x86/cpu: Cleanup the untrain mess

x86/cpu: Cleanup the untrain mess

commit e7c25c441e9e0fa75b4c83e0b26306b702cfe90d upstream.

Since there can only be one active return_thunk, there only needs be
one (matching) untrain_ret. It fundamentally doesn't make sense to
allow multiple untrain_ret at the same time.

Fold all the 3 different untrain methods into a single (temporary)
helper stub.

Fixes: fb3bd914b3ec ("x86/srso: Add a Speculative RAS Overflow mitigation")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230814121149.042774962@infradead.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>