nvmet-tcp: add an helper to free the cmd buffers

Makes the code easier to read and to debug.

Sets the freed pointers to NULL, it will be useful
when destroying the queues to understand if the commands'
buffers have been released already or not.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: John Meneghini <jmeneghi@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>