x86/srso: Fix unret validation dependencies

CONFIG_CPU_SRSO isn't dependent on CONFIG_CPU_UNRET_ENTRY (AMD
Retbleed), so the two features are independently configurable.  Fix
several issues for the (presumably rare) case where CONFIG_CPU_SRSO is
enabled but CONFIG_CPU_UNRET_ENTRY isn't.

Fixes: fb3bd914

 ("x86/srso: Add a Speculative RAS Overflow mitigation")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/299fb7740174d0f2335e91c58af0e9c242b4bac1.1693889988.git.jpoimboe@kernel.org