openvswitch: drop unneeded BUG_ON() in ovs_flow_cmd_build_info()

All the callers of ovs_flow_cmd_build_info() already deal with
error return code correctly, so we can handle the error condition
in a more gracefull way. Still dump a warning to preserve
debuggability.

v1 -> v2:
 - clarify the commit message
 - clean the skb and report the error (DaveM)

Fixes: ccb1352e ("net: Add Open vSwitch kernel components.")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>