IB/srpt: Fix srpt_close_session()

Avoid that srpt_close_session() waits if it doesn't have to wait.
Additionally, increase the time during which srpt_close_session()
waits until closing a session has finished. This makes it easier
to detect session shutdown bugs.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Cc: Alex Estrin <alex.estrin@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>