net: increase the size of priv_flags and add IFF_OVS_DATAPATH

IFF_OVS_DATAPATH is a place-holder for the Open vSwitch datapath
which I am preparing to submit for merging.

As all 16 bits of priv_flags are already assigned flags, also increase
the size of priv_flags to 32 bits.

Unfortunately, by my calculations this increases the size of
struct net_device by 4 bytes on 32bit architectures and
8 bytes on 64 bit architectures. I couldn't see an obvious
way to avoid that.

Cc: Jesse Gross <jesse@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>