quota: cleanup dquota sync functions (version 4)

Currently the VFS calls vfs_dq_sync to sync out disk quotas for a given
superblock.  This is a small wrapper around sync_dquots which for the
case of a non-NULL superblock is a small wrapper around quota_sync_sb.

Just make quota_sync_sb global (rename it to sync_quota_sb) and call it
directly.  Also call it directly for those cases in quota.c that have a
superblock and leave sync_dquots purely an iterator over sync_quota_sb and
remove it's superblock argument.

To make this nicer move the check for the lack of a quota_sync method
from the callers into sync_quota_sb.

[folded build fix from Alexander Beregalov <a.beregalov@gmail.com>]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/sync.c

@@ -27,7 +27,7 @@
  */
 static int __sync_filesystem(struct super_block *sb, int wait)
 {
-	vfs_dq_sync(sb);
+	sync_quota_sb(sb, -1);
 	sync_inodes_sb(sb, wait);
 	lock_super(sb);
 	if (sb->s_dirt && sb->s_op->write_super)