libceph: use swap() macro instead of taking tmp variable

Fix the following coccicheck warning:
net/ceph/crush/mapper.c:1077:8-9: WARNING opportunity for swap()

by using swap() for the swapping of variable values and drop
the tmp variable that is not needed any more.
Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/crush/mapper.c

@@ -906,7 +906,6 @@ int crush_do_rule(const struct crush_map *map,
 	int recurse_to_leaf;
 	int wsize = 0;
 	int osize;
-	int *tmp;
 	const struct crush_rule *rule;
 	__u32 step;
 	int i, j;
@@ -1073,9 +1072,7 @@ int crush_do_rule(const struct crush_map *map,
 				memcpy(o, c, osize*sizeof(*o));
 
 			/* swap o and w arrays */
-			tmp = o;
-			o = w;
-			w = tmp;
+			swap(o, w);
 			wsize = osize;
 			break;