x86/fault: Rename no_context() to kernelmode_fixup_or_oops()

The name no_context() has never been very clear.  It's only called for
faults from kernel mode, so rename it and change the no-longer-useful
user_mode(regs) check to a WARN_ON_ONCE.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/c21940efe676024bb4bc721f7d70c29c420e127e.1612924255.git.luto@kernel.org