btrfs: avoid monopolizing a core when activating a swap file

commit 2c8507c63f upstream.

During swap activation we iterate over the extents of a file and we can
have many thousands of them, so we can end up in a busy loop monopolizing
a core. Avoid this by doing a voluntary reschedule after processing each
extent.

CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/btrfs/inode.c

@@ -7128,6 +7128,8 @@ noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
 			ret = -EAGAIN;
 			goto out;
 		}
+
+		cond_resched();
 	}
 
 	btrfs_release_path(path);