RISC-V: Take text_mutex in ftrace_init_nop()

Without this we get lockdep failures.  They're spurious failures as SMP isn't
up when ftrace_init_nop() is called.  As far as I can tell the easiest fix is
to just take the lock, which also seems like the safest fix.

Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Acked-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>