xfs: split xfs_initialize_perag Factor out a xfs_perag_alloc helper that allocates a single perag structure. 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>