[PATCH] quota: sanitize dentry handling in vfs_quota_on_mount

Use lookup_one_len instead of opencoding a simplified lookup using
lookup_hash with a fake hash.

Also there's no need anymore for the d_invalidate as we have a completely
valid dentry now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>