arm64: force_signal_inject: WARN if called from kernel context

force_signal_inject() is designed to send a fatal signal to userspace,
so WARN if the current pt_regs indicates a kernel context. This can
currently happen for the undefined instruction trap, so patch that up so
we always BUG() if we didn't have a handler.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>