IB/srpt: Introduce srpt_disconnect_ch_sync()

Except for changing a BUG_ON() call into a WARN_ON_ONCE() call, this
patch does not change any functionality.

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