Merge remote-tracking branch 'xfrm: start adding netlink extack support'

Sabrina Dubroca says:

============
XFRM states and policies are complex objects, and there are many
reasons why the kernel can reject userspace's request to create
one. This series makes it a bit clearer by providing extended ack
messages for policy creation.

A few other operations that reuse the same helper functions are also
getting partial extack support in this series. More patches will
follow to complete extack support, in particular for state creation.

Note: The policy->share attribute seems to be entirely ignored in the
kernel outside of checking its value in verify_newpolicy_info(). There
are some (very) old comments in copy_from_user_policy and
copy_to_user_policy suggesting that it should at least be copied
to/from userspace. I don't know what it was intended for.
============
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>