xfs: don't override bc_ops for staging btrees

Add a few conditionals for staging btrees to the core btree code instead
of overloading the bc_ops vector.

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>