fsck.f2fs: fix to repair ro mounted device w/ -f

fsck.f2fs: fix to repair ro mounted device w/ -f

As Hagbard Celine reported:

"
Referring to the output from the fsck running against a "ro"
filesystem, especially this line:
Info: Check FS only due to RO

As far as i can tell this says that opposed to other filesystems
running fsck against a "ro" mounted f2fs partition will never fix any
errors.
So I tried running fsck against the same partition mounted "rw":
- mount -o remount,rw /mnt/f2fstest/
- fsck.f2fs  -f /dev/nvme0n1p7
Info: Force to fix corruption
Info: Mounted device!
        Error: Not available on mounted device!

I might be misunderstanding something, but all this tells me that
unless one make a custom initramfs that runs fsck before root is
mounted (something no distributions has, as far as I know), fsck will
never fix an f2fs formatted root partition during boot.
If this is by design and not a bug/unintended behavior, it should be
documented somewhere least more people will experience system crashes
like mine.

All tests above done with kernel 5.0.5 and f2fs-tools 1.12.0 with
"fsck.f2fs: allow to fsck readonly image w/ -f option"-patch by Chao
Yu.
"

We try to make our fsck behavior keeping line with e2fsprogs, but w/
-f option, we can just check a RO mounted device rather repair it, so
let's fix this.

Reported-and-Tested-by: Hagbard Celine <hagbardcelin@gmail.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>