CVE-2026-80775 futex: Fix race on the initial mm->futex.phash.ref allocation

In the Linux kernel, the following vulnerability has been resolved: futex: Fix race on the initial mm-futex.phash.ref allocation futexhashallocate allocates mm-futex.phash.ref without any locking. Commit d9b05321e21e "futex: Move futexhashfree back to mmput" moved the allocation here and assumed...