x86/fault: Skip the AMD erratum #91 workaround on unaffected CPUs

According to the Revision Guide for AMD Athlon™ 64 and AMD Opteron™
Processors, only early revisions of family 0xF are affected. This will
avoid unnecessarily fetching instruction bytes before sending SIGSEGV to
user programs.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/477173b7784bc28afb3e53d76ae5ef143917e8dd.1612924255.git.luto@kernel.org