x86/bugs: Do IBPB fallback check only once

commit 0fe4aeea upstream.

When booting with retbleed=auto, if the kernel wasn't built with
CONFIG_CC_HAS_RETURN_THUNK, the mitigation falls back to IBPB.  Make
sure a warning is printed in that case.  The IBPB fallback check is done
twice, but it really only needs to be done once.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>