quota: Properly invalidate caches even for filesystems with blocksize < pagesize

Sometimes invalidate_bdev() can fail to invalidate a part of block
device cache because of dirty data. If the filesystem has blocksize
smaller than page size, this can happen even for pages containing
quota files and thus kernel would operate on stale data. Fix the
issue by syncing the filesystem before invalidating the cache.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>