->reserved is either 0 or sizeof(struct rcu_head), can't be negative.

Link: http://lkml.kernel.org/r/20180305200730.15812-12-adobriyan@gmail.comSigned-off-by: NAlexey Dobriyan <adobriyan@gmail.com>
Acked-by: NChristoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>