Remove the topology lock from disk_gone(), it might be called with regular

Remove the topology lock from disk_gone(), it might be called with regular
mutexes held and the topology lock is an sx lock.

The topology lock was there to protect traversing through the list of providers
of disk's geom, but it seems that disk's geom has always exactly one provider.

Change the code to call g_wither_provider() for this one provider, which is
safe to do without holding the topology lock and assert that there is indeed
only one provider.

Discussed with:	ken
MFC after:	1 week