nvme-tcp: fix potential memory corruption in nvme_tcp_recv_pdu()

nvme_tcp_recv_pdu() doesn't check the validity of the header length.
When header digests are enabled, a target might send a packet with an
invalid header length (e.g. 255), causing nvme_tcp_verify_hdgst()
to access memory outside the allocated area and cause memory corruptions
by overwriting it with the calculated digest.

Fix this by rejecting packets with an unexpected header length.

Fixes: 3f2304f8 ("nvme-tcp: add NVMe over TCP host driver")
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>