UPSTREAM: xfrm/compat: Attach xfrm dumps to 64=>32 bit translator

Currently nlmsg_unicast() is used by functions that dump structures that
can be different in size for compat tasks, see dump_one_state() and
dump_one_policy().

The following nlmsg_unicast() users exist today in xfrm:

         Function                          |    Message can be different
                                           |       in size on compat
-------------------------------------------|------------------------------
    xfrm_get_spdinfo()                     |               N
    xfrm_get_sadinfo()                     |               N
    xfrm_get_sa()                          |               Y
    xfrm_alloc_userspi()                   |               Y
    xfrm_get_policy()                      |               Y
    xfrm_get_ae()                          |               N

Besides, dump_one_state() and dump_one_policy() can be used by filtered
netlink dump for XFRM_MSG_GETSA, XFRM_MSG_GETPOLICY.

Just as for xfrm multicast, allocate frag_list for compat skb journey
down to recvmsg() which will give user the desired skb according to
syscall bitness.

Signed-off-by: Dmitry Safonov <dima@arista.com>

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
(cherry picked from commit 5f3eea6b7e8f58cf5c8a9d4b9679dc19e9e67ba3)
Bug: 163141236
Signed-off-by: Alistair Delva <adelva@google.com>
Change-Id: Id1a606ddd9d7dfe73a448eeb252b1bfd8dbd2fcb