Merge branch 'possible_net_t'

Eric W. Biederman says:

====================
Introduce possible_net_t

The current usage of write_pnet and read_pnet is a little laborious and
error prone as you only notice if you failed to include them if are
compiling with network namespaces enabled.

possible_net_t remedies that by using a type that is 0 bytes when
network namespaces are disabled and can only be read and written to with
read_pnet and write_pnet.

Aka less work and safer for the same effect.

I kill hold_net and release_net first as are they are haven't been used
since 2008 and are noise at the points where write_pnet and read_pnet
are used.

I have folded in Eric Dumazets suggestions to improve the killing of
hold_net and release net.  And respon.  I had to respin anyway as
there was enough changes elsewhere in the tree the previous version
of these patches did not quite apply cleanly.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>