f2fs: do FG_GC when GC boosting is required for zoned devices

Under low free section count, we need to use FG_GC instead of BG_GC to
recover free sections.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>