fs: dlm: call dlm_lsop_recover_prep once

A lockspace can be "stopped" multiple times consecutively before
being "started" (when recoveries overlap.)  In this case, the
lsop_recover_prep callback only needs to be called once when the
lockspace is first stopped, and not repeatedly for each stop.

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