xfrm: ressurrect "Fix uninitialized memory read in _decode_session4"

xfrm: ressurrect "Fix uninitialized memory read in _decode_session4"

This resurrects commit ebffdb9e99a2e9e2b
("xfrm4: Fix uninitialized memory read in _decode_session4"),
which got lost during a merge conflict resolution between ipsec-next
and net-next tree.

1c288a48e097 ("xfrm: remove decode_session indirection from afinfo_policy")
in ipsec-next moved the (buggy) _decode_session4 from
net/ipv4/xfrm4_policy.c to net/xfrm/xfrm_policy.c.
In mean time, ebffdb9e99a2e was applied to ipsec.git and fixed the
problem in the "old" location.

When the trees got merged, the moved, old function was kept.
This applies the "lost" commit again, to the new location.

Fixes: 939a2be1d864c ("Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/klassert/ipsec-next")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>