dlm: clear ast_type when removing from astqueue

The lkb_ast_type field indicates whether the lkb is on the astqueue list.
When clearing locks for a process, lkb's were being removed from the astqueue
list without clearing the field.  If release_lockspace then happened
immediately afterward, it could try to remove the lkb from the list a second
time.

Appears when process calls libdlm dlm_release_lockspace() which first
closes the ls dev triggering clear_proc_locks, and then removes the ls
(a write to control dev) causing release_lockspace().

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