mlxsw: spectrum_dpipe: Take router lock from dpipe code

The dpipe code traverses internal router structures such as neighbours
and adjacency entries and dumps them to user space via netlink. Up until
now the routing code did not have its own locks and relied on RTNL lock
to serialize access. This is going to change with the introduction of
the router lock.

Take the router lock in the code paths where RTNL lock is currently
taken so that the latter could be removed by subsequent patches.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>