riscv: add do_page_fault and do_trap_break into the kprobes blacklist

These two functions are used to implement the kprobes feature so they
can't be kprobed.

Fixes: c22b0bcb

 ("riscv: Add kprobes supported")
Cc: stable@vger.kernel.org
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>