xen-netfront: fix xennet_release_tx_bufs()

After restore on ia64 xen domain, kernel panics as follows.
This patch fixes it.

union skb_entry assumes sizeof(link->skb, pointer) ==
sizeof(list->link, unsigned).
However this isn't true on ia64. So  make link type unsigned long.
And introduced two accesor.

kernel unaligned access to 0xe0000000000000bd, ip=0xa0000001004c2ca0
xenwatch[14]: error during unaligned kernel access
 -1 [1]
Modules linked in:

Pid: 14, CPU 0, comm:             xenwatch
psr : 0000101008422010 ifs : 8000000000000307 ip  : [<a0000001004c2ca0>]    Not tainted (2.6.26-rc4xen-ia64-dirty)
ip is at dev_kfree_skb_irq+0x20/0x1a0
unat: 0000000000000000 pfs : 400000000000040b rsc : 0000000000000007
rnat: 0000000000000000 bsps: 0000000000000000 pr  : 000000000000a941
ldrs: 0000000000000000 ccv : 0000000000000000 fpsr: 0009804c8a70433f
csd : 0000000000000000 ssd : 0000000000000000
b0  : a0000001003efb70 b6  : a000000100070e40 b7  : a000000100070e40
f6  : 1003e000000fcb75352b1 f7  : 1003e000000000014ff97
f8  : 1003e00fcb74fc3454d80 f9  : 1003e0000000080000000
f10 : 1003e0000000000001431 f11 : 1003e0000000000989680
r1  : a000000100bfcf80 r2  : e0000000000000bd r3  : 000000000000308c
r8  : 0000000000000000 r9  : e00000000fc31310 r10 : a000000100a13b28
r11 : 0000000000000000 r12 : e00000000fd0fdf0 r13 : e00000000fd08000
r14 : 0000000000000000 r15 : e00000000fcc8000 r16 : 0000000000000009
r17 : e000010000104000 r18 : e000010000104000 r19 : a000000100a13b40
r20 : a0000001009c23f0 r21 : a0000001009fd4d0 r22 : 0000000000004000
r23 : 0000000000000000 r24 : fffffffffff04c10 r25 : 0000000000000002
r26 : 0000000000000000 r27 : 0000000000000000 r28 : e00000000fd08bd4
r29 : a0000001007570b8 r30 : a0000001009e5500 r31 : a0000001009e54a0

Call Trace:
 [<a000000100026000>] show_stack+0x40/0xa0
                                sp=e00000000fd0f670 bsp=e00000000fd08f68
 [<a000000100026a60>] show_regs+0x9a0/0x9e0
                                sp=e00000000fd0f840 bsp=e00000000fd08f10
 [<a000000100037680>] die+0x260/0x3a0
                                sp=e00000000fd0f840 bsp=e00000000fd08ec8
 [<a000000100037810>] die_if_kernel+0x50/0x80
                                sp=e00000000fd0f840 bsp=e00000000fd08e98
 [<a00000010003eb40>] ia64_handle_unaligned+0x2ea0/0x2fc0
                                sp=e00000000fd0f840 bsp=e00000000fd08df0
 [<a00000010001ca30>] ia64_prepare_handle_unaligned+0x30/0x60
                                sp=e00000000fd0fa10 bsp=e00000000fd08df0
 [<a00000010005d100>] paravirt_leave_kernel+0x0/0x40
                                sp=e00000000fd0fc20 bsp=e00000000fd08df0
 [<a0000001004c2ca0>] dev_kfree_skb_irq+0x20/0x1a0
                                sp=e00000000fd0fdf0 bsp=e00000000fd08db8
 [<a0000001003efb70>] xennet_release_tx_bufs+0xd0/0x120
                                sp=e00000000fd0fdf0 bsp=e00000000fd08d78
 [<a0000001003f14c0>] backend_changed+0xc40/0xf80
                                sp=e00000000fd0fdf0 bsp=e00000000fd08d08
 [<a00000010034bd50>] otherend_changed+0x190/0x1c0
                                sp=e00000000fd0fe00 bsp=e00000000fd08cc8
 [<a000000100349530>] xenwatch_thread+0x310/0x3c0
                                sp=e00000000fd0fe00 bsp=e00000000fd08ca0
 [<a0000001000cb040>] kthread+0xe0/0x160
                                sp=e00000000fd0fe30 bsp=e00000000fd08c68
 [<a000000100024450>] kernel_thread_helper+0x30/0x60
                                sp=e00000000fd0fe30 bsp=e00000000fd08c40
 [<a00000010001a8a0>] start_kernel_thread+0x20/0x40
                                sp=e00000000fd0fe30 bsp=e00000000fd08c40
Kernel panic - not syncing: Aiee, killing interrupt handler!

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>