xfs: turn the allocbt cursor active field into a btree flag

Add a new XFS_BTREE_ALLOCBT_ACTIVE flag to replace the active field.

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>