datapath: Move sysfs support from brcompat_mod into openvswitch_mod.

datapath: Move sysfs support from brcompat_mod into openvswitch_mod.

In the past problems have arisen due to the different ways that datapaths
are created and destroyed in the three different cases:

	1. sysfs supported, brcompat_mod loaded.
	2. sysfs supported, brcompat_mod not loaded.
	3. sysfs not supported.

The brcompat_mod loaded versus not loaded distinction is the stickiest
because we have to do all the calls into brcompat_mod through hook
functions, which in turn causes pressure to keep the number of hook
functions small and well-defined, which makes it really difficult to put
the hook call points at exactly the right place.  Witness, for example,
this piece of code in datapath.c:

        int dp_del_port(struct net_bridge_port *p)
        {
                ASSERT_RTNL();

        #ifdef SUPPORT_SYSFS
                if (p->port_no != ODPP_LOCAL && dp_del_if_hook)
                        sysfs_remove_link(&p->dp->ifobj, p->dev->name);
        #endif

The code inside the #ifdef is logically part of the brcompat_mod sysfs
support, but the author of this code (quite reasonably) didn't want to
add a hook function call there.  After all, what would you call the
hook function?  There's no obvious name from the dp_del_port() caller's
perspective.

All this argues that sysfs support should be in openvswitch_mod itself,
since it has to be tightly integrated, not bolted on.  So this commit
moves it there.

Now, this is not to say that openvswitch_mod should actually be
implementing bridge-compatible sysfs.  In the future, it probably should
not be; rather, it should implement something appropriate for Open vSwitch
datapaths instead.  But right now we have bridge-compatible sysfs, and so
that's what this commit moves.