packet: bail out of packet_snd() if L2 header creation fails

Due to a misplaced parenthesis, the expression

  (unlikely(offset) < 0),

which expands to

  (__builtin_expect(!!(offset), 0) < 0),

never evaluates to true. Therefore, when sending packets with
PF_PACKET/SOCK_DGRAM, packet_snd() does not abort as intended
if the creation of the layer 2 header fails.

Spotted by Coverity - CID 1259975 ("Operands don't affect result").

Fixes: 9c707762 ("packet: make packet_snd fail on len smaller than l2 header")
Signed-off-by: Christoph Jaeger <cj@linux.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>