netfilter: prefer nla_strlcpy for dealing with NLA_STRING attributes

fixes these warnings:
'nfnl_cthelper_create' at net/netfilter/nfnetlink_cthelper.c:237:2,
'nfnl_cthelper_new' at net/netfilter/nfnetlink_cthelper.c:450:9:
./include/linux/string.h:246:9: warning: '__builtin_strncpy' specified bound 16 equals destination size [-Wstringop-truncation]
  return __builtin_strncpy(p, q, size);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Moreover, strncpy assumes null-terminated source buffers, but thats
not the case here.
Unlike strlcpy, nla_strlcpy *does* pad the destination buffer
while also considering nla attribute size.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>