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: cdd1b2bd358f ("riscv: kprobes: Implement alloc_insn_page()")
Signed-off-by: Jisheng Zhang <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Palmer Dabbelt <[e-mail]>

arch/riscv/kernel/probes/kprobes.c

···
 	return 0;
 }
 
+#ifdef CONFIG_MMU
 void *alloc_insn_page(void)
 {
 	return  __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START, VMALLOC_END,
···
 				     VM_FLUSH_RESET_PERMS, NUMA_NO_NODE,
 				     __builtin_return_address(0));
 }
+#endif
 
 /* install breakpoint in text */
 void __kprobes arch_arm_kprobe(struct kprobe *p)