vlan: Use bitmask of feature flags instead of seperate feature bits

Herbert Xu points out that the use of seperate feature bits for features
to be propagated to VLAN devices is going to get messy real soon.
Replace the VLAN feature bits by a bitmask of feature flags to be
propagated and restore the old GSO_SHIFT/MASK values.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>