bpf, sockmap: sk_skb data_end access incorrect when src_reg = dst_reg

The current conversion of skb->data_end reads like this:

  ; data_end = (void*)(long)skb->data_end;
   559: (79) r1 = *(u64 *)(r2 +200)   ; r1  = skb->data
   560: (61) r11 = *(u32 *)(r2 +112)  ; r11 = skb->len
   561: (0f) r1 += r11
   562: (61) r11 = *(u32 *)(r2 +116)
   563: (1f) r1 -= r11

But similar to the case in 84f44df6 ("bpf: sock_ops sk access may stomp
registers when dst_reg = src_reg"), the code will read an incorrect skb->len
when src == dst. In this case we end up generating this xlated code:

  ; data_end = (void*)(long)skb->data_end;
   559: (79) r1 = *(u64 *)(r1 +200)   ; r1  = skb->data
   560: (61) r11 = *(u32 *)(r1 +112)  ; r11 = (skb->data)->len
   561: (0f) r1 += r11
   562: (61) r11 = *(u32 *)(r1 +116)
   563: (1f) r1 -= r11

... where line 560 is the reading 4B of (skb->data + 112) instead of the
intended skb->len Here the skb pointer in r1 gets set to skb->data and the
later deref for skb->len ends up foll...