dlm: drop mutex use in waiters recovery

The waiters_mutex no longer needs to be used in the waiters recovery
functions dlm_recover_waiters_pre() and dlm_recover_waiters_pre().
During recovery, ordinary locking operations are paused, and the
recovery thread is the only context accessing the waiters list,
so the lock is not needed.

Access to the waiters list from debugfs functions is avoided by
taking the top level recovery lock in the debugfs dump function.

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