f2fs: adjust unusable cap before checkpoint=disable mode

The unusable cap value must be adjusted before checking whether
checkpoint=disable is feasible.

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