target: Un-export target_get_sess_cmd()

There are no in-tree users of target_get_sess_cmd() outside of
target_core_transport.c.  Any new code should use the higher-level
target_submit_cmd() interface.  So let's un-export target_get_sess_cmd()
and make it static to the one file where it's actually used.

(nab: Fix up minor fuzz to for-next)

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>