etf: Add skip_sock_check

Currently, etf expects a socket with SO_TXTIME option set for each packet
it encounters. So, it will drop all other packets. But, in the future
commits we are planning to add functionality where tstamp value will be set
by another qdisc. Also, some packets which are generated from within the
kernel (e.g. ICMP packets) do not have any socket associated with them.

So, this commit adds support for skip_sock_check. When this option is set,
etf will skip checking for a socket and other associated options for all
skbs.

Signed-off-by: Vedang Patel <vedang.patel@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>