xfs: factor out a xfs_btree_owner helper

Split out a helper to calculate the owner for a given btree instead of
duplicating the logic in two places.  While we're at it, make the
bc_ag/bc_ino switch logic depend on the correct geometry flag.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: break this up into two patches for the owner check]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>