fd-lk: implement new method to check for empty fd_lk_ctx_t

to use this in fuse-bridge, to conditionally attempt lock migration
to new graph.

Change-Id: I9b10d5839c4ac6051c75e4c020371391cd886ac8
BUG: 808400
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4455
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>