btrfs: relocation: Introduce error injection points for cancelling balance

btrfs: relocation: Introduce error injection points for cancelling balance

Introduce a new error injection point, should_cancel_balance().

It's just a wrapper of atomic_read(&fs_info->balance_cancel_req), but
allows us to override the return value.

Currently there are only one locations using this function:
- btrfs_balance()
  It checks cancel before each block group.

There are other locations checking fs_info->balance_cancel_req, but they
are not used as an indicator to exit, so there is no need to use the
wrapper.

But there will be more locations coming, and some locations can cause
kernel panic if not handled properly.

So introduce this error injection to provide better test interface.

Signed-off-by: Qu Wenruo <wqu@suse.com>
(cherry picked from commit 6d2751364569f885ca795e37882690a78ac9f149)