quota: Split dquot_quota_sync() to writeback and cache flushing part

Split off part of dquot_quota_sync() which writes dquots into a quota file
to a separate function. In the next patch we will use the function from
filesystems and we do not want to abuse ->quota_sync quotactl callback more
than necessary.

Acked-by: Steven Whitehouse <swhiteho@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/quotaops.h b/include/linux/quotaops.h
index 17b9773..ec6b65f 100644
--- a/include/linux/quotaops.h
+++ b/include/linux/quotaops.h
@@ -83,7 +83,8 @@
 int dquot_quota_on_mount(struct super_block *sb, char *qf_name,
  	int format_id, int type);
 int dquot_quota_off(struct super_block *sb, int type);
-int dquot_quota_sync(struct super_block *sb, int type, int wait);
+int dquot_writeback_dquots(struct super_block *sb, int type);
+int dquot_quota_sync(struct super_block *sb, int type);
 int dquot_get_dqinfo(struct super_block *sb, int type, struct if_dqinfo *ii);
 int dquot_set_dqinfo(struct super_block *sb, int type, struct if_dqinfo *ii);
 int dquot_get_dqblk(struct super_block *sb, int type, qid_t id,
@@ -255,6 +256,11 @@
 
 #define dquot_file_open		generic_file_open
 
+static inline int dquot_writeback_dquots(struct super_block *sb, int type)
+{
+	return 0;
+}
+
 #endif /* CONFIG_QUOTA */
 
 static inline int dquot_alloc_space_nodirty(struct inode *inode, qsize_t nr)