xfs: consolidate the xfs_alloc_lookup_* helpers

Add a single xfs_alloc_lookup helper to sort out the argument passing and
setting of the active flag instead of duplicating the logic three times.

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>