x86/bugs: Restructure SRSO mitigation

Restructure SRSO to use select/update/apply functions to create
consistent vulnerability handling.  Like with retbleed, the command line
options directly select mitigations which can later be modified.

While at it, remove a comment which doesn't apply anymore due to the
changed mitigation detection flow.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/20250418161721.1855190-17-david.kaplan@amd.com