LoongArch: BPF: Fix off-by-one error in build_prologue()

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

LoongArch: BPF: Fix off-by-one error in build_prologue()

Vincent reported that running BPF progs with tailcalls on LoongArch

causes kernel hard lockup. Debugging the issues shows that the JITed

image missing a jirl instruction at the end of the epilogue.

There are two passes in JIT compiling, the first pass set the flags and

the second pass generates JIT code based on those flags. With BPF progs

mixing bpf2bpf and tailcalls, build_prologue() generates N insns in the

first pass and then generates N+1 insns in the second pass. This makes

epilogue_offset off by one and we will jump to some unexpected insn and

cause lockup. Fix this by inserting a nop insn.