net: if: Release the interface lock early when rejoining mcast groups

In order to avoid any mutex deadlocks between iface->lock and
TX lock, release the interface lock before calling a function
that will acquire TX lock. See previous commit for similar issue
in RS timer handling. So here we create a separate list of multicast
addresses that are to be rejoined when network interface comes up
and then rejoin the groups without iface->lock held.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>