netfilter: nf_tables: Store user-defined hook ifname

Prepare for hooks with NULL ops.dev pointer (due to non-existent device)
and store the interface name and length as specified by the user upon
creation. No functional change intended.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>