ceph: fix possible double-free of mds request reference

Clear pointer to mds request after dropping the reference to
ensure we don't drop it again, as there is at least one error
path through this function that does not reset fi->last_readdir
to a new value.
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/dir.c

@@ -288,8 +288,10 @@ more:
 			CEPH_MDS_OP_LSSNAP : CEPH_MDS_OP_READDIR;
 
 		/* discard old result, if any */
-		if (fi->last_readdir)
+		if (fi->last_readdir) {
 			ceph_mdsc_put_request(fi->last_readdir);
+			fi->last_readdir = NULL;
+		}
 
 		/* requery frag tree, as the frag topology may have changed */
 		frag = ceph_choose_frag(ceph_inode(inode), frag, NULL, NULL);