dpll: rely on rcu for netdev_dpll_pin()

This fixes a possible UAF in if_nlmsg_size(),
which can run without RTNL.

Add rcu protection to "struct dpll_pin"

Move netdev_dpll_pin() from netdevice.h to dpll.h to
decrease name pollution.

Note: This looks possible to no longer acquire RTNL in
netdev_dpll_pin_assign() later in net-next.

v2: do not force rcu_read_lock() in rtnl_dpll_pin_size() (Jiri Pirko)

Fixes: 5f18426928 ("netdev: expose DPLL pin handle for netdevice")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Cc: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240223123208.3543319-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/dpll/dpll_core.c

@@ -564,7 +564,7 @@ void dpll_pin_put(struct dpll_pin *pin)
 		xa_destroy(&pin->parent_refs);
 		xa_erase(&dpll_pin_xa, pin->id);
 		dpll_pin_prop_free(&pin->prop);
-		kfree(pin);
+		kfree_rcu(pin, rcu);
 	}
 	mutex_unlock(&dpll_lock);
 }

drivers/dpll/dpll_core.h

@@ -47,6 +47,7 @@ struct dpll_device {
  * @prop:		pin properties copied from the registerer
  * @rclk_dev_name:	holds name of device when pin can recover clock from it
  * @refcount:		refcount
+ * @rcu:		rcu_head for kfree_rcu()
  **/
 struct dpll_pin {
 	u32 id;
@@ -57,6 +58,7 @@ struct dpll_pin {
 	struct xarray parent_refs;
 	struct dpll_pin_properties prop;
 	refcount_t refcount;
+	struct rcu_head rcu;
 };
 
 /**