samples/bpf: Enable powerpc support

Add the necessary definitions for building bpf samples on ppc.

Since ppc doesn't store function return address on the stack, modify how
PT_REGS_RET() and PT_REGS_FP() work.

Also, introduce PT_REGS_IP() to access the instruction pointer.

Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/samples/bpf/tracex2_kern.c b/samples/bpf/tracex2_kern.c
index 09c1adc..6d6eefd 100644
--- a/samples/bpf/tracex2_kern.c
+++ b/samples/bpf/tracex2_kern.c
@@ -27,10 +27,10 @@
 	long init_val = 1;
 	long *value;
 
-	/* x64/s390x specific: read ip of kfree_skb caller.
+	/* read ip of kfree_skb caller.
 	 * non-portable version of __builtin_return_address(0)
 	 */
-	bpf_probe_read(&loc, sizeof(loc), (void *)PT_REGS_RET(ctx));
+	BPF_KPROBE_READ_RET_IP(loc, ctx);
 
 	value = bpf_map_lookup_elem(&my_map, &loc);
 	if (value)