s390/bpf: Use bpf_skip() in bpf_jit_prologue()

Now that we have bpf_skip() for emitting nops, use it in
bpf_jit_prologue() in order to reduce code duplication.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200717165326.6786-6-iii@linux.ibm.com

arch/s390/net/bpf_jit_comp.c

@@ -519,10 +519,11 @@ static void bpf_jit_prologue(struct bpf_jit *jit, u32 stack_depth)
 		/* xc STK_OFF_TCCNT(4,%r15),STK_OFF_TCCNT(%r15) */
 		_EMIT6(0xd703f000 | STK_OFF_TCCNT, 0xf000 | STK_OFF_TCCNT);
 	} else {
-		/* j tail_call_start: NOP if no tail calls are used */
-		EMIT4_PCREL(0xa7f40000, 6);
-		/* bcr 0,%0 */
-		EMIT2(0x0700, 0, REG_0);
+		/*
+		 * There are no tail calls. Insert nops in order to have
+		 * tail_call_start at a predictable offset.
+		 */
+		bpf_skip(jit, 6);
 	}
 	/* Tail calls have to skip above initialization */
 	jit->tail_call_start = jit->prg;