datapath: Don't write into IPV4_TUNNEL data when using TUN_ID.

When the IPV4_TUNNEL action is executed, a pointer in the skb is
directly assigned the address of the action, which is protected by
RCU.  If a TUN_ID action is later executed it will write into the
action, which is not allowed by RCU.  This avoids the problem by
making a copy of the data and writing into the copy.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>