Add extra check to avoid skb buffer overflow. Fixes crash below:

 [  101.030427] ------------[ cut here ]------------
 [  101.030459] kernel BUG at net/core/skbuff.c:127!
 [  101.030486] invalid opcode: 0000 [#1] SMP
...
 [  101.030806] Pid: 2010, comm: btmrvl_main_ser Not tainted 3.5.0+ #80 Laptop
 [  101.030859] EIP: 0060:[<c14f2ba9>] EFLAGS: 00010282 CPU: 0
 [  101.030894] EIP is at skb_put+0x99/0xa0
 [  101.030919] EAX: 00000080 EBX: f129380b ECX: ef923540 EDX: 00000001
 [  101.030956] ESI: f00a4000 EDI: 00001003 EBP: ed4a5efc ESP: ed4a5ecc
 [  101.030992]  DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
 [  101.031024] CR0: 8005003b CR2: 08fca014 CR3: 30960000 CR4: 000407f0
 [  101.031062] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
 [  101.031100] DR6: ffff0ff0 DR7: 00000400
 [  101.031125] Process btmrvl_main_ser (pid: 2010, ti=ed4a4000 task=ef923540 task.ti=ed4a4000)
 [  101.031174] Stack:
 [  101.031188]  c18126f8 c1651938 f853f8d2 00001003 00001003 f1292800 f1292808 f129380b
 [  101.031250]  f1292940 f00a4000 eddb1280 efc0f9c0 ed4a5f44 f853f8d2 00000040 00000000
 [  101.031312]  ef923540 c15ee096 ef923540 eddb12d4 00000004 f00a4000 00000040 00000000
 [  101.031376] Call Trace:
 [  101.031396]  [<f853f8d2>] ? btmrvl_sdio_process_int_status+0x272/0x3d0 [btmrvl_sdio]
 [  101.031444]  [<f853f8d2>] btmrvl_sdio_process_int_status+0x272/0x3d0 [btmrvl_sdio]
 [  101.031488]  [<c15ee096>] ? _raw_spin_unlock_irqrestore+0x36/0x70
 [  101.031526]  [<f85a46e4>] btmrvl_service_main_thread+0x244/0x300 [btmrvl]
 [  101.031568]  [<f853fb50>] ? btmrvl_sdio_poll_card_status.isra.6.constprop.7+0x90/0x90 [btmrvl_sdio]
 [  101.031619]  [<c107eda0>] ? try_to_wake_up+0x270/0x270
 [  101.031648]  [<f85a44a0>] ? btmrvl_process_event+0x3b0/0x3b0 [btmrvl]
 [  101.031686]  [<c106d19d>] kthread+0x7d/0x90
 [  101.031713]  [<c106d120>] ? flush_kthread_work+0x150/0x150
 [  101.031745]  [<c15f5a82>] kernel_thread_helper+0x6/0x10
...
 [  101.032008] EIP: [<c14f2ba9>] skb_put+0x99/0xa0 SS:ESP 0068:ed4a5ecc
 [  101.056125] ---[ end trace a0bd01d1a9a796c8 ]---
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>