CVE-2026-64375: proc: protect ptrace_may_access() with exec_update_lock (FD links)

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

proc: protect ptracemayaccess() with execupdatelock (FD links)

procpidgetlink() and procpidreadlink() currently look up the task from the pid once, then do the ptrace access check on that task, then look up the task from the pid a second time to do the actual access. That's racy in several ways.

To fix it, pass the task to the ->procgetlink() handler, and instead of procfdaccessallowed(), introduce a new helper callprocgetlink() that looks up and locks the task, does the access check, and calls ->procgetlink().