dlm: Switch to using wait_event()

We saw an issue in a production server on a customer deployment where
DLM 4.0.7 gets "stuck" and unable to join new lockspaces.

There is no useful response for the dlm in do_event() if
wait_event_interruptible() is interrupted, so switch to
wait_event().

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: David Teigland <teigland@redhat.com>