iscsi-target: remove usage of ->shutdown_session

->shutdown session only decideѕ if the target core calls ->close_session
directly, or if the fabrics drivers calls it manually later through
target_put_session, which at this point will always close the session as
it has been removed from the lookup list and thus no new references will
be acquired from the core.

So instead remove ->shutdown and have the core call ->close_session
directly, and replace all calls to target_put_session in the iSCSI target
with direct calls to iscsit_close_session.

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