dpif-linux: Close channel Netlink sockets when a port number gets recycled.

dpif-linux: Close channel Netlink sockets when a port number gets recycled.

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 partially fixes the problem.  Now, if a port number gets
reused, add_channel() closes the old Netlink socket assigned to that port
before it installs the new one.

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