riscv: fix runtime constant support for nommu kernels

In the Linux kernel, the following vulnerability has been resolved:

riscv: fix runtime constant support for nommu kernels

the `__runtime_fixup_32` function does not handle the case where `val` is

zero correctly (as might occur when patching a nommu kernel and referring

to a physical address below the 4GiB boundary whose upper 32 bits are all

zero) because nothing in the existing logic prevents the code from taking

the `else` branch of both nop-checks and emitting two `nop` instructions.

This leaves random garbage in the register that is supposed to receive the

upper 32 bits of the pointer instead of zero that when combined with the

value for the lower 32 bits yields an invalid pointer and causes a kernel

panic when that pointer is eventually accessed.

The author clearly considered the fact that if the `lui` is converted into

a `nop` that the second instruction needs to be adjusted to become an `li`

instead of an `addi`, hence introducing the `addi_insn_mask` variable, but

didn't follow that logic through fully to the case where the `else` branch

executes. To fix it just adjust the logic to ensure that the second `else`

branch is not taken if the first instruction will be patched to a `nop`.