dpif-linux: Don't reset kernel upcall_pids unintentionally.

dpif-linux: Don't reset kernel upcall_pids unintentionally.

Commit b063d9f0 "datapath: Use unicast Netlink sockets for upcalls" that
introduced an 'upcall_pid' member into struct dpif_linux_vport, struct
dpif_linux_dp, and struct dpif_linux_flow neglected to do so only if the
member was nonzero.  This caused every datapath, vport, and flow operation
to supply an upcall_pid.  In particular, the netdev_set_config() called at
startup when a vport already existed caused the upcall_pid for that vport
to be reset to 0, which in turn caused all packets received on the vport to
be dropped instead of forwarded to ovs-vswitchd.

Reported-by: Shih-Hao Li <shli@nicira.com>
Bug #7714.