iflib publishes the interface before IFDI_ATTACH_POST, where a driver
seeds what these methods report, so all five return ENXIO until it has
run.

Setter requests are validated here, not in each driver. A table must
have isc_rss_table_size entries, the length iflib already masks flowids
with. Queries clear the buffer past the interface name, so a get
followed by a set does not fail on the caller's leftovers.

Signed-off-by: Nick Price <nprice@FreeBSD.org>