IB/srpt: Simplify srpt_close_session()

Move a mutex lock and unlock statement from srpt_close_session()
into srpt_disconnect_ch_sync(). Since the previous patch removed
the last user of the return value of that function, change the
return value of srpt_disconnect_ch_sync() into void.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>