p4-ebpf: Fix warnings generated by clang when compiling XDP helper program

[osinstom]

Currently, clang generates the following warnings when compiling XDP helper program:

out.c:313:26: warning: cast to 'struct ethhdr *' from smaller integer type '__u32' (aka 'unsigned int') [-Wint-to-pointer-cast] 
   struct ethhdr *eth = (struct ethhdr *) skb->data;
                        ^ 
out.c:329:60: warning: cast to 'void *' from smaller integer type '__u32' (aka 'unsigned int') [-Wint-to-void-pointer-cast]                                                                              
   if ((void *) ((struct internal_metadata *) meta + 1) > (void *) skb->data)
                                                          ^ 
2 warnings generated.   

They don't impact the functionality, but might be annoying for users. This PR fixes the XDP helper program.

[mestery]

LGTM