[XFS] Fix sparse warning in kmem_shake_allow

We can't return a masked result of a __bitwise type. Compare it to 0 first
to keep the behaviour without the warning.

SGI-PV: 968555
SGI-Modid: xfs-linux-melb:xfs-kern:29309a

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