i825xx: fix paging fault on znet_probe()

In znet_probe(), strncmp() may access beyond 0x100000 and
trigger the below oops in kvm.  Fix it by limiting the loop
under 0x100000-8. I suspect the limit could be further decreased
to 0x100000-sizeof(struct netidblk), however no datasheet at hand..

[    3.744312] BUG: unable to handle kernel paging request at 80100000
[    3.746145] IP: [<8119d12a>] strncmp+0xc/0x20
[    3.747446] *pde = 01d10067 *pte = 00100160
[    3.747493] Oops: 0000 [#1] DEBUG_PAGEALLOC
[    3.747493] Pid: 1, comm: swapper Not tainted 3.6.0-rc1-00018-g57bfc0a #73 Bochs Bochs
[    3.747493] EIP: 0060:[<8119d12a>] EFLAGS: 00010206 CPU: 0
[    3.747493] EIP is at strncmp+0xc/0x20
[    3.747493] EAX: 800fff4e EBX: 00000006 ECX: 00000006 EDX: 814d2bb9
[    3.747493] ESI: 80100000 EDI: 814d2bba EBP: 8e03dfa0 ESP: 8e03df98
[    3.747493]  DS: 007b ES: 007b FS: 0000 GS: 00e0 SS: 0068
[    3.747493] CR0: 8005003b CR2: 80100000 CR3: 016f7000 CR4: 00000690
[    3.747493] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
[    3.747493] DR6: ffff0ff0 DR7: 00000400
[    3.747493] Process swapper (pid: 1, ti=8e03c000 task=8e040000 task.ti=8e03c000)
[    3.747493] Stack:
[    3.747493]  800fffff 00000000 8e03dfb4 816a1376 00000006 816a134a 00000000 8e03dfd0
[    3.747493]  816819b5 816ed1c0 8e03dfe4 00000006 00000123 816ed604 8e03dfe4 81681b29
[    3.747493]  00000000 81681a5b 00000000 00000000 8134e542 00000000 00000000 00000000
[    3.747493] Call Trace:
[    3.747493]  [<816a1376>] znet_probe+0x2c/0x26b
[    3.747493]  [<816a134a>] ? dnet_driver_init+0xf/0xf
[    3.747493]  [<816819b5>] do_one_initcall+0x6a/0x110
[    3.747493]  [<81681b29>] kernel_init+0xce/0x14b

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>