net: xfrm: allow clearing socket xfrm policies.

Currently it is possible to add or update socket policies, but
not clear them. Therefore, once a socket policy has been applied,
the socket cannot be used for unencrypted traffic.

This patch allows (privileged) users to clear socket policies by
passing in a NULL pointer and zero length argument to the
{IP,IPV6}_{IPSEC,XFRM}_POLICY setsockopts. This results in both
the incoming and outgoing policies being cleared.

The simple approach taken in this patch cannot clear socket
policies in only one direction. If desired this could be added
in the future, for example by continuing to pass in a length of
zero (which currently is guaranteed to return EMSGSIZE) and
making the policy be a pointer to an integer that contains one
of the XFRM_POLICY_{IN,OUT} enum values.

An alternative would have been to interpret the length as a
signed integer and use XFRM_POLICY_IN (i.e., 0) to clear the
input policy and -XFRM_POLICY_OUT (i.e., -1) to clear the output
policy.

Tested: https://android-review.googlesource.com/539816


Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>