nvmet-tcp: fix memory leak when performing a controller reset

If a reset controller is executed while the initiator
is performing some I/O the driver may leak the memory allocated
for the commands' iovec.

Make sure that nvmet_tcp_uninit_data_in_cmds() releases
all the memory.

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>