riscv: cfi: reject unknown flags in PR_SET_CFI

riscv: cfi: reject unknown flags in PR_SET_CFI

prctl(PR_SET_CFI,PR_CFI_BRANCH_LANDING_PADS) silently ignored
unknown control values. Only PR_CFI_{ENABLE,DISABLE,LOCK} should
be permitted.

This changes the behavior of the uABI (fails previously accepted bits
with EINVAL).

Fixes: 08ee1559052b ("prctl: cfi: change the branch landing pad prctl()s to be more descriptive")
Signed-off-by: Richard Patel <ripatel@wii.dev>
Link: https://patch.msgid.link/20260518183918.322545-1-ripatel@wii.dev
[pjw@kernel.org: change the patch description to note that although this is a uABI change, it does not break the uABI]
Signed-off-by: Paul Walmsley <pjw@kernel.org>