xfs: add a name field to struct xfs_btree_ops

The btnum in struct xfs_btree_ops is often used for printing a symbolic
name for the btree.  Add a name field to the ops structure and use that
directly.

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