f2fs: make f2fs_read_inline_data() more readable

In f2fs_read_inline_data(), it is confused with checking of
inline_data flag, as we checked it before calling. So this
patch add some comments for f2fs_has_inline_data().

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