librbd: avoid losing sparseness in read_parent()

librbd: avoid losing sparseness in read_parent()

When read_parent() constructs a read for image_ctx->parent, it employs
a thick bufferlist (either re-using the bufferlist on the object extent
or creating a temporary one inside of C_ObjectReadMergedExtents).  This
forgoes any sparseness: even if the result obtained by ObjectRequest is
sparse, it's thickened by ReadResult's handler for Bufferlist type.

This behavior is very old and hasn't been a problem for regular clones
because the public API returns a thick bufferlist in the case of C++ or
equivalent char* buf/struct iovec iov[] buffers in the case of C anyway.
ObjectCacher isn't sparse-aware but it's also not used for caching reads
by default and reading from parent for the purposes of a copyup is done
in CopyupRequest in a way that preserves sparseness.  However, when it
comes to migration, source image reads go through read_parent() and the
destination image gets thickened as an inadvertent side effect.

Fix this by introducing a new ChildObject type for ReadResult whose
handler would plant the result obtained by parent's ObjectRequest into
child's ObjectRequest, as if read_parent() wasn't even called.

Fixes: https://tracker.ceph.com/issues/73831
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>