xfrm: ressurrect "Fix uninitialized memory read in _decode_session4"

This resurrects commit 8742dc86
("xfrm4: Fix uninitialized memory read in _decode_session4"),
which got lost during a merge conflict resolution between ipsec-next
and net-next tree.

c53ac41e ("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, 8742dc86 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: a658a3f2 ("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>