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>