various: fix finding metadata inode numbers when metadir is enabled

various: fix finding metadata inode numbers when metadir is enabled

There are a number of tests that use xfs_db to examine the contents of
metadata inodes to check correct functioning.  The logic is scattered
everywhere and won't work with metadata directory trees, so make a
shared helper to find these inodes.

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