dpif-linux: Fix fd leak for vports that disappear from the datapath.

dpif-linux: Fix fd leak for vports that disappear from the datapath.

When ovs-vswitchd deletes a port with dpif_linux_port_del(), that function
uses del_channel() to delete the corresponding channel, including closing
its Netlink socket fd.  However, if the vport gets removed by some other
process (e.g. "ip link delete" for veths) then this function never gets
called and thus the channel never gets deleted.

This commit fixes the problem by resynchronizing channels with the
datapath whenever a vport gets deleted.

Bug #16784.
Reported-by: Paul Ingram <paul@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>