powerpc/bpf/32: No need to zeroise r4 when not doing tail call

r4 is cleared at function entry and used as tail call count.

But when the function does not perform tail call, r4 is
ignored, so no need to clear it.

Replace it by a NOP in that case.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9c5440b2b6d90a78600257433ac499b5c5101fbb.1675245773.git.christophe.leroy@csgroup.eu

arch/powerpc/net/bpf_jit_comp32.c

@@ -114,7 +114,10 @@ void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
 	int i;
 
 	/* Initialize tail_call_cnt, to be skipped if we do tail calls. */
-	EMIT(PPC_RAW_LI(_R4, 0));
+	if (ctx->seen & SEEN_TAILCALL)
+		EMIT(PPC_RAW_LI(_R4, 0));
+	else
+		EMIT(PPC_RAW_NOP());
 
 #define BPF_TAILCALL_PROLOGUE_SIZE	4