MFC r355451: Remove some branching from GEOM_DISK hot path.

pp->private just can not be NULL in those places.

In g_disk_start() and g_disk_ioctl() both dp != NULL and !dp->d_destroyed
should always be true if disk_gone() and disk_destroy() are used properly,
since GEOM does not send requests to errored providers. If the protocol is
not followed, then no amount of additional checks here give real safety.

In g_disk_access() though the checks are useful, since GEOM blocks only
new opens for errored providers, but allows closes. It should not happen
if disk_gone() and disk_destroy() are used properly, but may otherwise.

To improve cases when disk_gone() is not used, call it from disk_destroy().
It does not give full guaranties, but it errors the provider and makes
GEOM block unwanted requests at least after some race.