KEYS: Reject keyring creation with overridden credentials

If the current task is running with overridden credentials calling
commit_creds() is forbidden and triggers a BUG_ON().

Reject keyring creation with -EPERM when credentials are overridden.

Cc: stable@vger.kernel.org # v5.10+
Suggested-by: Jann Horn <jannh@google.com>
Closes: https://lore.kernel.org/keyrings/CAG48ez0XjVSR=M--UBauTm8sJuc+ZbhKzaPa3a6wrSVHyoKevw@mail.gmail.com/
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
v3:
Collapse and open-code cred_overridden() to the entry point of lookup_user_key() (David).
v2:
- Added mandatory tags.
- No code changes.
- Link: https://lore.kernel.org/keyrings/20261019145624.309790-2-jarkko@kernel.org/
 Documentation/security/keys/core.rst | 3 ++-
 security/keys/process_keys.c         | 6 +++++-
 2 files changed, 7 insertions(+), 2 deletions(-)