s390/bpf: Use bcr 0,%0 as tail call nop filler

Currently used 0x0000 filler confuses bfd disassembler, making bpftool
prog dump xlated output nearly useless. Fix by using a real instruction.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200602174555.2501389-1-iii@linux.ibm.com

arch/s390/net/bpf_jit_comp.c

@@ -503,7 +503,8 @@ static void bpf_jit_prologue(struct bpf_jit *jit, u32 stack_depth)
 	} else {
 		/* j tail_call_start: NOP if no tail calls are used */
 		EMIT4_PCREL(0xa7f40000, 6);
-		_EMIT2(0);
+		/* bcr 0,%0 */
+		EMIT2(0x0700, 0, REG_0);
 	}
 	/* Tail calls have to skip above initialization */
 	jit->tail_call_start = jit->prg;