xfs: initialize btree blocks using btree_ops structure

xfs: initialize btree blocks using btree_ops structure

Source kernel commit: c87e3bf7802477cb4500dfafe0ab039313aa2dda

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>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>