ip_tunnel: fix ipv4 pmtu check to honor inner ip header df

ip_tunnel: fix ipv4 pmtu check to honor inner ip header df

Frag needed should be sent only if the inner header asked
to not fragment. Currently fragmentation is broken if the
tunnel has df set, but df was not asked in the original
packet. The tunnel's df needs to be still checked to update
internally the pmtu cache.

Commit a3fe0f108e053514 broke it, and this commit fixes
the ipv4 df check back to the way it was.

Fixes: a3fe0f108e053514 ("ip_tunnels: Use skb-len to PMTU check.")
Cc: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>