bonding: Ensure that we unshare skbs prior to calling pskb_may_pull

Recently reported oops:

kernel BUG at net/core/skbuff.c:813!
invalid opcode: 0000 [#1] SMP
last sysfs file: /sys/devices/virtual/net/bond0/broadcast
CPU 8
Modules linked in: sit tunnel4 cpufreq_ondemand acpi_cpufreq freq_table bonding
ipv6 dm_mirror dm_region_hash dm_log cdc_ether usbnet mii serio_raw i2c_i801
i2c_core iTCO_wdt iTCO_vendor_support shpchp ioatdma i7core_edac edac_core bnx2
ixgbe dca mdio sg ext4 mbcache jbd2 sd_mod crc_t10dif mptsas mptscsih mptbase
scsi_transport_sas dm_mod [last unloaded: microcode]

Modules linked in: sit tunnel4 cpufreq_ondemand acpi_cpufreq freq_table bonding
ipv6 dm_mirror dm_region_hash dm_log cdc_ether usbnet mii serio_raw i2c_i801
i2c_core iTCO_wdt iTCO_vendor_support shpchp ioatdma i7core_edac edac_core bnx2
ixgbe dca mdio sg ext4 mbcache jbd2 sd_mod crc_t10dif mptsas mptscsih mptbase
scsi_transport_sas dm_mod [last unloaded: microcode]
Pid: 0, comm: swapper Not tainted 2.6.32-71.el6.x86_64 #1 BladeCenter HS22
-[7870AC1]-
RIP: 0010:[<ffffffff81405b16>]  [<ffffffff81405b16>]
pskb_expand_head+0x36/0x1e0
RSP: 0018:ffff880028303b70  EFLAGS: 00010202
RAX: 0000000000000002 RBX: ffff880c6458ec80 RCX: 0000000000000020
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff880c6458ec80
RBP: ffff880028303bc0 R08: ffffffff818a6180 R09: ffff880c6458ed64
R10: ffff880c622b36c0 R11: 0000000000000400 R12: 0000000000000000
R13: 0000000000000180 R14: ffff880c622b3000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff880028300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
CR2: 00000038653452a4 CR3: 0000000001001000 CR4: 00000000000006e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process swapper (pid: 0, threadinfo ffff8806649c2000, task ffff880c64f16ab0)
Stack:
 ffff880028303bc0 ffffffff8104fff9 000000000000001c 0000000100000000
<0> ffff880000047d80 ffff880c6458ec80 000000000000001c ffff880c6223da00
<0> ffff880c622b3000 0000000000000000 ffff880028303c10 ffffffff81407f7a
Call Trace:
<IRQ>
 [<ffffffff8104fff9>] ? __wake_up_common+0x59/0x90
 [<ffffffff81407f7a>] __pskb_pull_tail+0x2aa/0x360
 [<ffffffffa0244530>] bond_arp_rcv+0x2c0/0x2e0 [bonding]
 [<ffffffff814a0857>] ? packet_rcv+0x377/0x440
 [<ffffffff8140f21b>] netif_receive_skb+0x2db/0x670
 [<ffffffff8140f788>] napi_skb_finish+0x58/0x70
 [<ffffffff8140fc89>] napi_gro_receive+0x39/0x50
 [<ffffffffa01286eb>] ixgbe_clean_rx_irq+0x35b/0x900 [ixgbe]
 [<ffffffffa01290f6>] ixgbe_clean_rxtx_many+0x136/0x240 [ixgbe]
 [<ffffffff8140fe53>] net_rx_action+0x103/0x210
 [<ffffffff81073bd7>] __do_softirq+0xb7/0x1e0
 [<ffffffff810d8740>] ? handle_IRQ_event+0x60/0x170
 [<ffffffff810142cc>] call_softirq+0x1c/0x30
 [<ffffffff81015f35>] do_softirq+0x65/0xa0
 [<ffffffff810739d5>] irq_exit+0x85/0x90
 [<ffffffff814cf915>] do_IRQ+0x75/0xf0
 [<ffffffff81013ad3>] ret_from_intr+0x0/0x11
 <EOI>
 [<ffffffff8101bc01>] ? mwait_idle+0x71/0xd0
 [<ffffffff814cd80a>] ? atomic_notifier_call_chain+0x1a/0x20
 [<ffffffff81011e96>] cpu_idle+0xb6/0x110
 [<ffffffff814c17c8>] start_secondary+0x1fc/0x23f

Resulted from bonding driver registering packet handlers via dev_add_pack and
then trying to call pskb_may_pull. If another packet handler (like for AF_PACKET
sockets) gets called first, the delivered skb will have a user count > 1, which
causes pskb_may_pull to BUG halt when it does its skb_shared check.  Fix this by
calling skb_share_check prior to the may_pull call sites in the bonding driver
to clone the skb when needed.  Tested by myself and the reported successfully.

Signed-off-by: Neil Horman
CC: Andy Gospodarek <andy@greyhouse.net>
CC: Jay Vosburgh <fubar@us.ibm.com>
CC: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>