md-cluster: use DLM_LSFL_SOFTIRQ for dlm_new_lockspace()

Use the recently added DLM_LSFL_SOFTIRQ flag in dlm_new_lockspace(),
signalling the ability to handle callbacks being run from softirq
context.  The md-cluster callback functions only call complete(),
which is suitable for softirq.  This should make dlm lock request
completions more efficient by avoiding the workqueue context switch.

Acked-by: Heming Zhao <heming.zhao@suse.com>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>