Sparse now warns about comparing pointers to 0, so change all instance
where that happens to NULL instead.

SGI-PV: 968555
SGI-Modid: xfs-linux-melb:xfs-kern:29308a
Signed-off-by: NChristoph Hellwig <hch@infradead.org>
Signed-off-by: NDavid Chinner <dgc@sgi.com>
Signed-off-by: NTim Shimmin <tes@sgi.com>