librbd: fix split() for SparseExtent and SparseBufferlistExtent

librbd: fix split() for SparseExtent and SparseBufferlistExtent

SparseExtents and SparseBufferlist are typedefs for interval_map.  In
both cases, split() handler is broken: for the former the extent isn't
actually split and for the latter incorrect bufferlist is attached to
the split extent.

Fortunately, both SnapshotDelta as produced by ObjectListSnapsRequest
and SparseBufferlist used in a couple of places seem to be collections
where only disjoint intervals are inserted and splitting doesn't occur
(at least in the common case).  But still, this is a landmine waiting
for someone to step on it.

Fixes: https://tracker.ceph.com/issues/64423
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>