CVE-2026-98281: futex: Also allocate private hash on vfork()

In the Linux kernel, the following vulnerability has been resolved:

futex: Also allocate private hash on vfork()

As Jann demonstrated, it is entirely feasible to access the mm through vfork(). Therefore we need to allocate a private hash on vfork() as well as any other CLONEVM user.

Specifically, it must be avoided to have (private) futex waiters before allocating the private hash.