dev_get_by_index is being called in addr_resolve
function which returns NULL and NULL pointer access
leads to kernel crash.

Following call trace is observed while running
rdma_lat test application

[  146.173149] BUG: unable to handle kernel NULL pointer dereference
at 00000000000004a0
[  146.173198] IP: addr_resolve+0x9e/0x3e0 [ib_core]
[  146.173221] PGD 0 P4D 0
[  146.173869] Oops: 0000 [#1] SMP PTI
[  146.182859] CPU: 8 PID: 127 Comm: kworker/8:1 Tainted: G  O 4.15.0-rc6+ #18
[  146.183758] Hardware name: LENOVO System x3650 M5: -[8871AC1]-/01KN179,
 BIOS-[TCE132H-2.50]- 10/11/2017
[  146.184691] Workqueue: ib_cm cm_work_handler [ib_cm]
[  146.185632] RIP: 0010:addr_resolve+0x9e/0x3e0 [ib_core]
[  146.186584] RSP: 0018:ffffc9000362faa0 EFLAGS: 00010246
[  146.187521] RAX: 000000000000001b RBX: ffffc9000362fc08 RCX:
0000000000000006
[  146.188472] RDX: 0000000000000000 RSI: 0000000000000096 RDI
: ffff88087fc16990
[  146.189427] RBP: ffffc9000362fb18 R08: 00000000ffffff9d R09:
00000000000004ac
[  146.190392] R10: 00000000000001e7 R11: 0000000000000001 R12:
ffff88086af2e090
[  146.191361] R13: 0000000000000000 R14: 0000000000000001 R15:
00000000ffffff9d
[  146.192327] FS:  0000000000000000(0000) GS:ffff88087fc00000(0000)
knlGS:0000000000000000
[  146.193301] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  146.194274] CR2: 00000000000004a0 CR3: 000000000220a002 CR4:
00000000003606e0
[  146.195258] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[  146.196256] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7:
0000000000000400
[  146.197231] Call Trace:
[  146.198209]  ? rdma_addr_register_client+0x30/0x30 [ib_core]
[  146.199199]  rdma_resolve_ip+0x1af/0x280 [ib_core]
[  146.200196]  rdma_addr_find_l2_eth_by_grh+0x154/0x2b0 [ib_core]

The below patch adds the missing NULL pointer check
returned by dev_get_by_index before accessing the netdev to
avoid kernel crash.

We observed the below crash when we try to do the below test.

 server                       client
 ---------                    ---------
 |1.1.1.1|<----rxe-channel--->|1.1.1.2|
 ---------                    ---------

On server: rdma_lat -c -n 2 -s 1024
On client:rdma_lat 1.1.1.1 -c -n 2 -s 1024

Fixes: 20029832 ("IB/core: Validate route when we init ah")
Signed-off-by: Muneendra <muneendra.kumar@broadcom.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>