ext4: yield during large unlinks

During large unlink operations on files with extents, we can use a lot
of CPU time.  This adds a cond_resched() call when starting to examine
the next level of a multi-level extent tree.  Multi-level extent trees
are rare in the first place, and this should rarely be executed.
    
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/extents.c

@@ -2835,6 +2835,9 @@ again:
 				err = -EIO;
 				break;
 			}
+			/* Yield here to deal with large extent trees.
+			 * Should be a no-op if we did IO above. */
+			cond_resched();
 			if (WARN_ON(i + 1 > depth)) {
 				err = -EIO;
 				break;