fsck.f2fs: check /proc/mounts first to detect readonly

Once f2fs_stop_checkpoint makes f2fs as readonly, only /proc/mounts shows
RDONLY whereas /etc/mtab does not.

Change-Id: I389cc1589147fcf4da8822de32cd3313d881e65f
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
1 file changed