xfs: kill xfs_ialloc_pagi_init()

This is just a basic wrapper around xfs_ialloc_read_agi(), which can
be entirely handled by xfs_ialloc_read_agi() by passing a NULL
agibpp....

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>