xfs: fix incorrect test in xfs_alloc_ag_vextent_lastblock

When I lifted the code in xfs_alloc_ag_vextent_lastblock out of a loop,
I forgot to convert all the accesses to len to be pointer dereferences.

Coverity-id: 1457918
Fixes: 5113f8ec

 ("xfs: clean up weird while loop in xfs_alloc_ag_vextent_near")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>