btrfs: relocation: allow signal to cancel balance

Although btrfs balance can be canceled with "btrfs balance cancel"
command, it's still almost muscle memory to press Ctrl-C to cancel a
long running btrfs balance.

So allow btrfs balance to check signal to determine if it should exit.
The cancellation points are in known location and we're only adding one
more reason, so this should be safe.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -2656,7 +2656,8 @@ int setup_extent_mapping(struct inode *inode, u64 start, u64 end,
  */
 int btrfs_should_cancel_balance(struct btrfs_fs_info *fs_info)
 {
-	return atomic_read(&fs_info->balance_cancel_req);
+	return atomic_read(&fs_info->balance_cancel_req) ||
+		fatal_signal_pending(current);
 }
 ALLOW_ERROR_INJECTION(btrfs_should_cancel_balance, TRUE);