riscv: kprobes: Fix build error when MMU=n

lkp reported a randconfig failure:

arch/riscv/kernel/probes/kprobes.c:90:22: error: use of undeclared identifier 'PAGE_KERNEL_READ_EXEC'

We implemented the alloc_insn_page() to allocate PAGE_KERNEL_READ_EXEC
page for kprobes insn page for STRICT_MODULE_RWX. But if MMU=n, we
should fall back to the generic weak alloc_insn_page() by generic
kprobe subsystem.

Fixes: cdd1b2bd

 ("riscv: kprobes: Implement alloc_insn_page()")
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>