f2fs: add an ioctl() to explicitly trigger fsck later

This adds an option in ioctl(F2FS_IOC_SHUTDOWN) in order to trigger fsck by
setting a NEED_FSCK flag.

Generally, shutdown is used for the test to validate filesystem consistency, and
setting NEED_FSCK flag can be used for Android to trigger fsck.f2fs at boot time
explicitly so that we could measure the elapsed time as well as force filesystem
check.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>