md-cluster: fix missing memory free

There are several places we allocate dlm_lock_resource, but not free it.

leave() need free a lock resource too (from Guoqing)
Cc: Goldwyn Rodrigues <rgoldwyn@suse.com>
Cc: Guoqing Jiang <gqjiang@suse.com>
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>