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>
3 files changed