dlm: don't specify WQ_UNBOUND for the ast callback workqueue

This patch removes the WQ_UNBOUND flag (which implies WQ_HIGHPRI)
from the DLM's ast work queue, in favor of just WQ_HIGHPRI.
This has been shown to cause a 19 percent performance increase for
simultaneous inode creates on GFS2 with fs_mark.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>