I just read it. do_trap_no_signal presumably calls fixup_exceptionOh. I agree with you, and if a #BR from the kernel it should unconditionally call die.
because #UD uses it and #UD needs that handling. (I'm guessing that
there is actually a legitimate use for a kernel fixup on #UD somewhere
-- there's probably something that isn't covered by cpuid.)
There should not be a #BR from the kernel using the fixup mechanism.
IMO if the exception comes from the kernel, it should unconditionally
call die.