rbd: allocate image object names with a slab allocator
The names of objects used for image object requests are always fixed
size. So create a slab cache to manage them. Define a new function
rbd_segment_name_free() to match rbd_segment_name() (which is what
supplies the dynamically-allocated name buffer).
This is part of:
http://tracker.ceph.com/issues/3926
Signed-off-by: Alex Elder <elder@inktank.com> Reviewed-by: Josh Durgin <josh.durgin@inktank.com>