btrfs: noinline btrfs_should_cancel_balance

I was attempting to reproduce a problem that Zygo hit, but my error
injection wasn't firing for a few of the common calls to
btrfs_should_cancel_balance.  This is because the compiler decided to
inline it at these spots.  Keep this from happening by explicitly
marking the function as noinline so that error injection will always
work.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -2615,7 +2615,7 @@ int setup_extent_mapping(struct inode *inode, u64 start, u64 end,
 /*
  * Allow error injection to test balance cancellation
  */
-int btrfs_should_cancel_balance(struct btrfs_fs_info *fs_info)
+noinline int btrfs_should_cancel_balance(struct btrfs_fs_info *fs_info)
 {
 	return atomic_read(&fs_info->balance_cancel_req) ||
 		fatal_signal_pending(current);