net: dsa: get and put tree reference counting

Provide convenient dsa_tree_get and dsa_tree_put functions scoping a DSA
tree used to increment and decrement its reference counter, instead of
poking directly its kref structure.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>