target: simplify transport_generic_remove

Instead of duplicating the code from transport_release_fe_cmd re-use it by
allowing transport_release_fe_cmd to return wether it actually freed the
command or not.  Also rename transport_release_fe_cmd to transport_put_cmd
and add a kerneldoc comment for it to make the use case more obvious.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>