netdev-genl: fix error codes when outputting XDP features

-EINVAL will interrupt the dump. The correct error to return
if we have more data to dump is -EMSGSIZE.

Discovered by doing:

  for i in `seq 80`; do ip link add type veth; done
  ./cli.py --dbg-small-recv 5300 --spec netdev.yaml --dump dev-get >> /dev/null
  [...]
     nl_len = 64 (48) nl_flags = 0x0 nl_type = 19
     nl_len = 20 (4) nl_flags = 0x2 nl_type = 3
  	error: -22

Fixes: d3d854fd

 ("netdev-genl: create a simple family for netdev stuff")
Reviewed-by: Amritha Nambiar <amritha.nambiar@intel.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240613213044.3675745-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>