dlm: switch to GFP_ATOMIC in dlm allocations

Replace GFP_NOFS with GFP_ATOMIC.  Also stop using idr_preload which
uses a non-bh spin_lock.  This is further preparation for softirq
message processing.

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