dlm: use spin_lock_bh for message processing

Use spin_lock_bh for all spinlocks involved in message processing,
in preparation for softirq message processing.  DLM lock requests
from user space involve dlm processing in user context, in addition
to the standard kernel context, necessitating bh variants.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>