net: move net_device priv_flags out from UAPI

These are private to userspace, and they're unstable
anyway and can be shuffled at will (see 080e4130)
so any userspace application relying on them is on crack.

Test compiled with allyesconfig.

mcgrof@drvbp1 /pub/mem/mcgrof/net-next (git::master)$ make allyesconfig
mcgrof@drvbp1 /pub/mem/mcgrof/net-next (git::master)$ time make -j 20
...
  BUILD   arch/x86/boot/bzImage
Setup is 16992 bytes (padded to 17408 bytes).
System is 56153 kB
CRC 721d2751
Kernel: arch/x86/boot/bzImage is ready  (#1)
real    19m35.744s
user    280m37.984s
sys     27m54.104s

Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>