BUILD: cache: fix a build warning with gcc < 7

BUILD: cache: fix a build warning with gcc < 7

Gcc before 7 does really not like direct operations on cast pointers
such as "((struct a*)b)->c += d;". It turns our that we have exactly
that construct in 3.0 since commit 5baa9ea168 ("MEDIUM: cache: Save
body size of cached objects and track it on delivery").

It's generally sufficient to use an intermediary variable such as :
"({ (struct a*) _ = b; _; })->c +=d;" but that's ugly. Fortunately
DISGUISE() implicitly does something very similar and works fine, so
let's use that.

No backport is needed.