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.