We don't check for the validity of the lengths in the packet received
from the firmware.  If the MPDU length received in the rx descriptor
is too short to contain the header length and the crypt length
together, we may end up trying to copy a negative number of bytes
(headlen - hdrlen < 0) which will underflow and cause us to try to
copy a huge amount of data.  This causes oopses such as this one:

BUG: unable to handle kernel paging request at ffff896be2970000
PGD 5e201067 P4D 5e201067 PUD 5e205067 PMD 16110d063 PTE 8000000162970161
Oops: 0003 [#1] PREEMPT SMP NOPTI
CPU: 2 PID: 1824 Comm: irq/134-iwlwifi Not tainted 4.19.33-04308-geea41cf4930f #1
Hardware name: [...]
RIP: 0010:memcpy_erms+0x6/0x10
Code: 90 90 90 90 eb 1e 0f 1f 00 48 89 f8 48 89 d1 48 c1 e9 03 83 e2 07 f3 48 a5 89 d1 f3 a4 c3 66 0f 1f 44 00 00 48 89 f8 48 89 d1 <f3> a4 c3
 0f 1f 80 00 00 00 00 48 89 f8 48 83 fa 20 72 7e 40 38 fe
RSP: 0018:ffffa4630196fc60 EFLAGS: 00010287
RAX: ffff896be2924618 RBX: ffff896bc8ecc600 RCX: 00000000fffb4610
RDX: 00000000fffffff8 RSI: ffff896a835e2a38 RDI: ffff896be2970000
RBP: ffffa4630196fd30 R08: ffff896bc8ecc600 R09: ffff896a83597000
R10: ffff896bd6998400 R11: 000000000200407f R12: ffff896a83597050
R13: 00000000fffffff8 R14: 0000000000000010 R15: ffff896a83597038
FS:  0000000000000000(0000) GS:ffff896be8280000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff896be2970000 CR3: 000000005dc12002 CR4: 00000000003606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 iwl_mvm_rx_mpdu_mq+0xb51/0x121b [iwlmvm]
 iwl_pcie_rx_handle+0x58c/0xa89 [iwlwifi]
 iwl_pcie_irq_rx_msix_handler+0xd9/0x12a [iwlwifi]
 irq_thread_fn+0x24/0x49
 irq_thread+0xb0/0x122
 kthread+0x138/0x140
 ret_from_fork+0x1f/0x40

Fix that by checking the lengths for correctness and trigger a warning
to show that we have received wrong data.
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>