xfs: initialize btree blocks using btree_ops structure

Notice now that the btree ops structure encodes btree geometry flags and
the magic number through the buffer ops.  Refactor the btree block
initialization functions to use the btree ops so that we no longer have
to open code all that.

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