CVE-2026-43109

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

x86: shadow stacks: proper error handling for mmap lock

김영민 reports that shstk_pop_sigframe() doesn't check for errors from
mmap_read_lock_killable(), which is a silly oversight, and also shows
that we haven't marked those functions with "__must_check", which would
have immediately caught it.

So let's fix both issues.