f2fs: check the generic conditions first

Let's return errors caught by the generic checks. This fixes generic/494 where
it expects to see EBUSY by setattr_prepare instead of EINVAL by f2fs for active
swapfile.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>