bpf: sockmap, fix uninitialized variable

There is a potential execution path in which variable err is
returned without being properly initialized previously.

Fix this by initializing variable err to 0.

Addresses-Coverity-ID: 1468964 ("Uninitialized scalar variable")
Fixes: e5cd3abc ("bpf: sockmap, refactor sockmap routines to work with hashmap")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>