riscv: ftrace: support PREEMPT

Now, we can safely enable dynamic ftrace with kernel preemption.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20250407180838.42877-9-andybnac@gmail.com


Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>