[XFS] Fix sparse NULL vs 0 warnings

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: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>