xfs: fix incorrect test in xfs_alloc_ag_vextent_lastblock

xfs: fix incorrect test in xfs_alloc_ag_vextent_lastblock

Source kernel commit: 77ca1eed5a7d2bf0905562eb1a15aac76bc19fe4

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: 5113f8ec3753ed ("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>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>