kernel: backport DSA patches fixing null-pointer dereference for 5.10

Backport patches
  381a730182f1 ("net: dsa: Move VLAN filtering syncing out of dsa_switch_bridge_leave")
  108dc8741c20 ("net: dsa: Avoid cross-chip syncing of VLAN filtering")
from upstream (currently in net-next) to fix null-pointer dereference.

Signed-off-by: Marek Behún <kabel@kernel.org>