netfilter: nft_compat: use nfnetlink_unicast()

Use nfnetlink_unicast() which already translates EAGAIN to ENOBUFS,
since EAGAIN is reserved to report missing module dependencies to the
nfnetlink core.

e0241ae6

 ("netfilter: use nfnetlink_unicast() forgot to update
this spot.

Reported-by: Yajun Deng <yajun.deng@linux.dev>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>