f2fs: fix to clean previous mount option when remount_fs

In manual of mount, we descript remount as below:

"mount -o remount,rw /dev/foo /dir
After  this call all old mount options are replaced and arbitrary stuff from
fstab is ignored, except the loop= option which is internally generated and
maintained by the mount command."

Previously f2fs do not clear up old mount options when remount_fs, so we have no
chance of disabling previous option (e.g. flush_merge). Fix it.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>