quota: turn quotas off when remounting read-only

Turn off quotas before filesystem is remounted read only.  Otherwise quota
will try to write to read-only filesystem which does no good...  We could
also just refuse to remount ro when quota is enabled but turning quota off
is consistent with what we do on umount.
Signed-off-by: NJan Kara <jack@suse.cz>
Cc: <stable@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/super.c

@@ -604,6 +604,7 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
 			mark_files_ro(sb);
 		else if (!fs_may_remount_ro(sb))
 			return -EBUSY;
+		DQUOT_OFF(sb);
 	}
 
 	if (sb->s_op->remount_fs) {