rbd: don't create sysfs entries for non-mapped snapshots

rbd: don't create sysfs entries for non-mapped snapshots

When an rbd image gets mapped a device entry gets created for it
under /sys/bus/rbd/devices/<id>/.  Inside that directory there are
sysfs files that contain information about the image: its size,
feature bits, major device number, and so on.

Additionally, if that image has any snapshots, a device entry gets
created for each of those as a "child" of the mapped device.  Each
of these is a subdirectory of the mapped device, and each directory
contains a few files with information about the snapshot (its
snapshot id, size, and feature mask).

There is no clear benefit to having those device entries for the
snapshots.  The information provided via sysfs of of little real
value--and all of it is available via rbd CLI commands.  If we
still wanted to see the kernel's view of this information it could
be done much more simply by including it in a single sysfs file for
the mapped image.

But there *is* a clear cost to supporting them.  Every time a snapshot
context changes, these entries need to be updated (deleted snapshots
removed, new snapshots created).  The rbd driver is notified of
changes to the snapshot context via callbacks from an osd, and care
must be taken to coordinate removal of snapshot data structures
with the possibility of one these notifications occurring.

Things would be considerably simpler if we just didn't have to
maintain device entries for the snapshots.

So get rid of them.

The ability to map a snapshot of an rbd image will remain; the only
thing lost will be the ability to query these sysfs directories for
information about snapshots of mapped images.

This resolves:
    http://tracker.ceph.com/issues/4796

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>