xfs: check metadata directory file path connectivity

Create a new scrubber type that checks that well known metadata
directory paths are connected to the metadata inode that the incore
structures think is in use.  For example, check that "/quota/user" in
the metadata directory tree actually points to
mp->m_quotainfo->qi_uquotaip->i_ino.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>