perf/core: Fix refcount bug and potential UAF in perf_mmap

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UBUNTU-CVE-2026-23248

In the Linux kernel, the following vulnerability has been resolved: perf/core: Fix refcount bug and potential UAF in perfmmap Syzkaller reported a refcountt: addition on 0; use-after-free warning in perfmmap. The issue is caused by a race condition between a failing mmap setup and a concurrent mm...