LinuxKPI: skbuff: re-add LP64 check

Re-add one LP64 check for case 2 (36bit upper limit to contigmalloc)
to fix the build.
contigmalloc(9) takes a vm_paddr_t argument but on armv7 and some powerpc
we are still at 32bit there so the constant value would be changed
from long long to uint (-Wconstant-conversion).

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reported by: CI
Fixes: e6c91f8e5be66