datapath: Don't recursively sample packets or reset their "tun_id"s.

datapath: Don't recursively sample packets or reset their "tun_id"s.

execute_actions() is called recursively when ODPAT_SET_DL_TCI adds a VLAN
header to a GSO packet, but we don't want to re-sample the sub-packet or
re-reset its tun_id, so break those two actions into a wrapper function.

This commit mostly moves code around without modifying it.

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