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

fs/f2fs/super.c

@@ -618,6 +618,9 @@ static int f2fs_remount(struct super_block *sb, int *flags, char *data)
 	org_mount_opt = sbi->mount_opt;
 	active_logs = sbi->active_logs;
 
+	sbi->mount_opt.opt = 0;
+	sbi->active_logs = NR_CURSEG_TYPE;
+
 	/* parse mount options */
 	err = parse_options(sb, data);
 	if (err)