dlm: improve how bast mode handling

The lkb bastmode value is set in the context of processing the
lock, and read by the dlm_astd thread.  Because it's accessed
in these two separate contexts, the writing/reading ought to
be done under a lock.  This is simple to do by setting it and
reading it when the lkb is added to and removed from dlm_astd's
callback list which is properly locked.

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