IB/core: Fix possible crash to access NULL netdev

resolved_dev returned might be NULL as ifindex is transient number.
Ignoring NULL check of resolved_dev might crash the kernel.
Therefore perform NULL check before accessing resolved_dev.

Additionally rdma_resolve_ip_route() invokes addr_resolve() which
performs check and address translation for loopback ifindex.
Therefore, checking it again in rdma_resolve_ip_route() is not helpful.
Therefore, the code is simplified to avoid IFF_LOOPBACK check.

Fixes: 20029832 ("IB/core: Validate route when we init ah")
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>