net: dsa: provide a find or new tree helper

Rename dsa_get_dst to dsa_tree_find since it doesn't increment the
reference counter, rename dsa_add_dst to dsa_tree_alloc for symmetry
with dsa_tree_free, and provide a convenient dsa_tree_touch function to
find or allocate a new tree.

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>