xfs: kill xfs_alloc_pagf_init()

Trivial wrapper around xfs_alloc_read_agf(), can be easily replaced
by passing a NULL agfbp to xfs_alloc_read_agf().

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