nvmet-tcp: add bounds check on Transfer Tag

ttag is used as an index to get cmd in nvmet_tcp_handle_h2c_data_pdu(),
add a bounds check to avoid out-of-bounds access.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>