quota: make dquot_quota_sync return errors from ->sync_fs

Strangely, dquot_quota_sync ignores the return code from the ->sync_fs
call, which means that quotacalls like Q_SYNC never see the error.  This
doesn't seem right, so fix that.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <brauner@kernel.org>