CVE-2026-53243 rseq: Fix using an uninitialized stack variable in rseq_exit_user_update()

In the Linux kernel, the following vulnerability has been resolved: rseq: Fix using an uninitialized stack variable in rseqexituserupdate There is an bug in which an uninitialized stack variable is used in rseqexituserupdate as reported by syzbot: BUG: KMSAN: kernel-infoleak in rseqsetidsgetcsadd...