dlm: make posix locks interruptible

Replace wait_event_killable with wait_event_interruptible
so that a program waiting for a posix lock can be
interrupted by a signal.  With the killable version,
a program was not interruptible by a signal if it
had a signal handler set for it, overriding the default
action of terminating the process.

Signed-off-by: Eric Ren <zren@suse.com>
Signed-off-by: David Teigland <teigland@redhat.com>