xfrm: fix fragmentation for ipv4 xfrm tunnel

When generating the ip header for the transformed packet we just copy
the frag_off field of the ip header from the original packet to the ip
header of the new generated packet. If we receive a packet as a chain
of fragments, all but the last of the new generated packets have the
IP_MF flag set. We have to mask the frag_off field to only keep the
IP_DF flag from the original packet. This got lost with git commit
36cf9acf

 ("[IPSEC]: Separate
inner/outer mode processing on output")

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>