IB/cm: Remove now useless rcu_lock in dst_fetch_ha

This lock used to be protecting a call to dst_get_neighbour_noref,
however the below commit changed it to dst_neigh_lookup which no longer
requires rcu.

Access to nud_state, neigh_event_send or rdma_copy_addr does not require
RCU, so delete the lock.

Fixes: 02b61955 ("infiniband: Convert dst_fetch_ha() over to dst_neigh_lookup().")
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>