ice: add support for transmitting unreadable frags

Advertise netmem Tx support in ice. The only change needed is to set
ICE_TX_BUF_FRAG conditionally, only when skb_frag_is_net_iov() is
false. Otherwise, the Tx buffer type will be ICE_TX_BUF_EMPTY and
the driver will skip the DMA unmapping operation.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>