net: marvell: prestera: Refactor get/put VR functions

* Use refcount, instead of uint
* Increment/decrement recount inside get/put
* Fix error path in __prestera_vr_create. Remove unnecessary kfree.
* Make __prestera_vr_destroy symmetric to "create"

Fixes: bca5859b

 ("net: marvell: prestera: add hardware router objects accounting")
Signed-off-by: Yevhen Orlov <yevhen.orlov@plvision.eu>
Link: https://lore.kernel.org/r/20220111011014.4418-1-yevhen.orlov@plvision.eu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>