dpif-linux: Avoid duplicate code in dpif_linux_vport_send().

dpif-linux: Avoid duplicate code in dpif_linux_vport_send().

dpif_linux_vport_send() had duplicated most of the code in
dpif_linux_execute() in order to execute output actions in the
kernel.  This forces developers to remember to change both
functions whenever the kernel interface changes.  In particular,
commit 80e5eed9 "datapath: Get packet metadata from userspace in
odp_packet_cmd_execute()." broke netdev_linux_vport_send().  This
commit reorganizes the code and fixes the regression.

Bug #5818.