bpf: fix a return in sockmap_get_from_fd()

"map" is a valid pointer.  We wanted to return "err" instead.  Also
let's return a zero literal at the end.

Fixes: 174a79ff ("bpf: sockmap with sk redirect support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>