dpif-linux: Generalize dpif_linux_recv_set() to synchronize channels.

dpif-linux: Generalize dpif_linux_recv_set() to synchronize channels.

Until now, the code in dpif_linux_recv_set() has assumed it is starting
from a clean slate with no channels at all.  This commit generalizes it to
compare the existing set of channels against the ones that should exist
given the vports that the kernel has, adding any new ones that are missing
and removing any that should not exist.  A followup commit will make good
use of this functionality.

Signed-off-by: Ben Pfaff <blp@nicira.com>