ovs-brcompatd: Don't consume CPU if can't connect to config db

ovs-brcompatd: Don't consume CPU if can't connect to config db

When ovs-brcompatd can't connect to the database, the "ovs" variable
is never set.  The function "brc_recv_update" takes care of draining
brcompat kernel module's netlink messages.  When the netlink message
comes in to modify the bridge, that function never gets called, so a
netlink message always appears to be ready and we consume 100% CPU
looping.

With this commit, we log a warning and drop the request on the floor.

Bug #2373