datapath: Simplify make_writable().

datapath: Simplify make_writable().

The current implementation of make_writable() is both overly complex
and unnecessarily aggressive about copying data.  We can improve
performance by only making a copy of the data if someone else holds
a reference to the portion of the data that we want to modify.  This
means that if a clone is held by the TCP stack for retransmission then
we do not need to make a copy if we are changing the IP header because
it will get regenerated on retransmit anyways.  Even when it is necessary
to copy we avoid duplicating struct sk_buff.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>