datapath: Enforce mutual exclusion between bridge and brcompat_mod.

datapath: Enforce mutual exclusion between bridge and brcompat_mod.

In kernels 2.6.36, Open vSwitch can (and does) safely coexist with the
Linux bridge module, but it does not make sense to load both bridge and
brcompat_mod at the same time.  Until now, nothing has prevented both from
loading; when they both load, confusion (at best) results.  This fixes
the problem by enforcing mutual exclusion.

Bug #9226.
Launchpad bug #917309.
Reported-by: Rogério Vinhal Nunes
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>