bpf: fix a return in sockmap_get_from_fd()

"map" is a valid pointer.  We wanted to return "err" instead.  Also
let's return a zero literal at the end.

Fixes: 174a79ff ("bpf: sockmap with sk redirect support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/syscall.c

@@ -1125,11 +1125,11 @@ static int sockmap_get_from_fd(const union bpf_attr *attr, int ptype)
 		fdput(f);
 		bpf_prog_put(prog1);
 		bpf_prog_put(prog2);
-		return PTR_ERR(map);
+		return err;
 	}
 
 	fdput(f);
-	return err;
+	return 0;
 }
 
 static int bpf_prog_attach(const union bpf_attr *attr)