target: make ->shutdown_session optional

Turns out the template and thus many drivers got the return value wrong:
0 means the fabrics driver needs to put a session reference, which no
driver except for the iSCSI target drivers did.  Fortunately none of these
drivers supports explicit Node ACLs, so the bug was harmless.

Even without that only qla2xxx and iscsi every did real work in
shutdown_session, so get rid of the boilerplate code in all other
drivers.

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