RDMA/core: Protect against changing dst->dev during destination resolve

During resolving address process, during route lookup and while performing
src address translation in case of loopback mode, hold the rcu lock so
that if netdevice is moving to different net namespace, or being
unregistered, it can be synchronized with net/core/dev.c, ie

change_net_namespace()
->dev_close_many()
  ->rt6_uncached_list_flush_dev() who would change dst->dev

to loopback device of the given net namespace.

Therefore, hold the rcu lock and sync with synchronize_net() of
change_net_namespace() to ensure that netdevice cannot get freed while
dst->dev is being used.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>