dlm: introduce DLM_LSFL_SOFTIRQ_SAFE

Introduce a new external lockspace flag DLM_LSFL_SOFTIRQ_SAFE.  A
lockspace user will set this flag if it can handle dlm running the
callback functions from softirq context.  When not set, dlm will
continue to run callback functions from the dlm_callback workqueue.
The new lockspace flag cannot be used for user space lockspaces, so
a uapi placeholder definition is used for the new flag value.

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