ext4: use swap macro in mext_page_double_lock

Make use of the swap macro and remove unnecessary variable *tmp*.
This makes the code easier to read and maintain.

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/move_extent.c

@@ -134,9 +134,7 @@ mext_page_double_lock(struct inode *inode1, struct inode *inode2,
 		mapping[0] = inode1->i_mapping;
 		mapping[1] = inode2->i_mapping;
 	} else {
-		pgoff_t tmp = index1;
-		index1 = index2;
-		index2 = tmp;
+		swap(index1, index2);
 		mapping[0] = inode2->i_mapping;
 		mapping[1] = inode1->i_mapping;
 	}