xfrm4: Fix uninitialized memory read in _decode_session4

We currently don't reload pointers pointing into skb header
after doing pskb_may_pull() in _decode_session4(). So in case
pskb_may_pull() changed the pointers, we read from random
memory. Fix this by putting all the needed infos on the
stack, so that we don't need to access the header pointers
after doing pskb_may_pull().

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>