librbd: make has_parent() prone to callers from copyup

librbd: make has_parent() prone to callers from copyup

This is required when CopyupRequest would need to invoke
pre_object_map_update() as part of upcoming changes to
create fewer child image objects whenever possible.

CopyupRequest constructor accepts image extents as an
rvalue forcing the caller to transfer ownership to it
and leaving the original variable in an unspecified
stated making has_parent() return incorrect state when
invoked from CopyupRequest. Therefore, introduce a
private tracking state that can be used in place of
checking emptiness of parent image extents.

Signed-off-by: Venky Shankar <vshankar@redhat.com>