BACKPORT: f2fs: expose # of overprivision segments

This is useful when checking conditions during checkpoint=disable in Android.

Bug: 330501081
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit ed5309ad137e1f467ca1e1b61be3dc39d144316c)
[hakan: Resolved minor conflict in Documentation/ABI/testing/sysfs-fs-f2fs]
Signed-off-by: Håkan Kvist <hakan.kvist@sony.com>
Change-Id: I0bd59501836fbe8fe361da2872ae660f3ac1f541