xfs: support repairing metadata btrees rooted in metadir inodes

Adapt the repair code so that we can stage a new btree in the data fork
area of a metadir inode and reap the old blocks.  We already have nearly
all of the infrastructure; the only parts that were missing were the
metadata inode reservation handling.

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