ibmveth: Fix opps during MTU change on an active device

This fixes the following opps which can occur when trying to deallocate
receive buffer pools when changing the MTU of an active ibmveth device.

Oops: Kernel access of bad area, sig: 11 [#1]
NIP: d000000004db00e8 LR: d000000004db00ac CTR: 0000000000591038
REGS: c00000007fff39d0 TRAP: 0300   Not tainted  (2.6.36-rc1)
MSR: 8000000000009032 <EE,ME,IR,DR>  CR: 22248244  XER: 00000002
DAR: 0000000000000488, DSISR: 0000000042000000
TASK = c00000007c463790[6531] 'netserver' THREAD: c00000007a154000 CPU: 0
GPR00: 0000000000000000 c00000007fff3c50 d000000004dbd360 0000000000000001
GPR04: 0000000000000001 1fffffffffffffff 000000000000043c c00000007a8e9f60
GPR08: c00000007a8e9e20 0000000000000245 0000000000000488 0000000000000000
GPR12: 00000000000000c0 c000000006d70000 c00000007bfec098 c00000007bfebc2c
GPR16: c00000007a157c78 0000000000000000 0000000000000001 0000000000000000
GPR20: 0000000000000001 0000000000000010 c000000000b51180 c00000007a8e9d90
GPR24: c00000007a8e9da0 c00000007a8e9580 00000000000005ea 00000000000002ff
GPR28: 0000000000000004 0000000000000080 c000000000a946f8 c00000007a8e9d80
NIP [d000000004db00e8] .ibmveth_remove_buffer_from_pool+0xe8/0x130 [ibmveth]
LR [d000000004db00ac] .ibmveth_remove_buffer_from_pool+0xac/0x130 [ibmveth]
Call Trace:
[c00000007fff3c50] [d000000004db00ac] .ibmveth_remove_buffer_from_pool+0xac/0x130 [ibmveth] (unreliable)
[c00000007fff3cf0] [d000000004db31dc] .ibmveth_poll+0x30c/0x460 [ibmveth]
[c00000007fff3dd0] [c00000000042c4b8] .net_rx_action+0x178/0x278
[c00000007fff3eb0] [c000000000093cf0] .__do_softirq+0x118/0x1f8
[c00000007fff3f90] [c00000000002ab3c] .call_do_softirq+0x14/0x24
[c00000007a157600] [c00000000000e3e4] .do_softirq+0xec/0x110
[c00000007a1576a0] [c000000000093394] .local_bh_enable_ip+0xb4/0xe0
[c00000007a157720] [c0000000004f0bac] ._raw_spin_unlock_bh+0x3c/0x50
[c00000007a157790] [c0000000004186e0] .release_sock+0x158/0x188
[c00000007a157840] [c000000000479660] .tcp_recvmsg+0x560/0x9b8
[c00000007a157970] [c0000000004a0d78] .inet_recvmsg+0x80/0xd8
[c00000007a157a00] [c000000000413e28] .sock_recvmsg+0x128/0x178
[c00000007a157bf0] [c0000000004164ac] .SyS_recvfrom+0xb4/0x148
[c00000007a157d70] [c000000000411f3c] .SyS_socketcall+0x274/0x360
[c00000007a157e30] [c0000000000085b4] syscall_exit+0x0/0x40

Reported-by: Rafael Camarda Silva Folco <rfolco@linux.vnet.ibm.com>
Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Acked-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>