x86: shadow stacks: proper error handling for mmap lock

[ Upstream commit 52f657e34d7b21b47434d9d8b26fa7f6778b63a0 ]

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.

BUG=b/510709511
TEST=presubmit
RELEASE_NOTE=Fixed CVE-2026-43109 in the Linux kernel.

cos-patch: security-moderate
Reported-by: 김영민 <osori@hspace.io>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Change-Id: I419488f6509304cea0709cf8a6a0f5d86c4233b0
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kernel CVE Triage Automation <cloud-image-kernel-cve-triage-automation@prod.google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/152983
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Dom Huh <domhuh@google.com>
Reviewed-by: Miri Amarilio <mirilio@google.com>