net: fix ip_len in reassembled IP datagram

For some reason, the ip_len field in a reassembled IP datagram is set
to just the size of the payload, but it should be set to the value it
would have had if the datagram had never been fragmented in the first
place, i.e. size of payload plus size of IP header.

That latter value is currently returned correctly via the "len"
variable. And before entering net_defragment(), len does have the
value ntohs(ip->ip_len), so if we're not dealing with a
fragment (so net_defragment leaves *len alone), that relationship of
course also holds after the net_defragment() call.

The only use I can find of ip->ip_len after the net_defragment call is
the ntohs(ip->udp_len) > ntohs(ip->ip_len) sanity check - none of the
functions that are passed the "ip" pointer themselves inspect ->ip_len
but instead use the passed len.

But that sanity check is a bit odd, since the RHS really should be
"ntohs(ip->ip_len) - 20", i.e. the IP payload size.

Now that we've fixed things so that len == ntohs(ip->ip_len) in all
cases, change that sanity check to use len-20 as the RHS.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>