genetlink: don't merge dumpit split op for different cmds into single iter

Currently, split ops of doit and dumpit are merged into a single iter
item when they are subsequent. However, there is no guarantee that the
dumpit op is for the same cmd as doit op.

Fix this by checking if cmd is the same for both.
This problem does not occur in existing families.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20231021112711.660606-2-jiri@resnulli.us


Signed-off-by: Jakub Kicinski <kuba@kernel.org>