libnftables: Flush iface cache after command execution

libnftables: Flush iface cache after command execution

Commit 94a945ffa81b7 ("libnftables: Get rid of explicit cache flushes")
was a bit too optimistic in that it missed the remaining need to flush
interface cache after each command in interactive mode - otherwise,
newly added interfaces won't be recognized.

Although cli.c only calls nft_run_cmd_from_buffer(), flush caches in
nft_run_cmd_from_filename() as well for matters of consistency.

Fixes: 94a945ffa81b7 ("libnftables: Get rid of explicit cache flushes")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>