bpf/verifier: improve disassembly of BPF_NEG instructions

BPF_NEG takes only one operand, unlike the bulk of BPF_ALU[64] which are
 compound-assignments.  So give it its own format in print_bpf_insn().

Signed-off-by: Edward Cree <ecree@solarflare.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>