net, sockmap: Don't call bpf_prog_put() on NULL pointer

If bpf_prog_inc_not_zero() fails for skb_parser, then bpf_prog_put() is
called unconditionally on skb_verdict, even though it may be NULL. Fix
and tidy up error path.

Fixes: 743df8b7 ("bpf, sockmap: Check skb_verdict and skb_parser programs explicitly")
Addresses-Coverity-ID: 1497799: Null pointer dereferences (FORWARD_NULL)
Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20201012170952.60750-1-alex.dewar90@gmail.com