linux/dim: Fix divide by 0 in RDMA DIM

linux/dim: Fix divide by 0 in RDMA DIM

Fix a divide 0 error in rdma_dim_stats_compare() when prev->cpe_ratio ==
0.

CallTrace:
  Hardware name: H3C R4900 G3/RS33M2C9S, BIOS 2.00.37P21 03/12/2020
  task: ffff880194b78000 task.stack: ffffc90006714000
  RIP: 0010:backport_rdma_dim+0x10e/0x240 [mlx_compat]
  RSP: 0018:ffff880c10e83ec0 EFLAGS: 00010202
  RAX: 0000000000002710 RBX: ffff88096cd7f780 RCX: 0000000000000064
  RDX: 0000000000000000 RSI: 0000000000000002 RDI: 0000000000000001
  RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000000 R12: 000000001d7c6c09
  R13: ffff88096cd7f780 R14: ffff880b174fe800 R15: 0000000000000000
  FS:  0000000000000000(0000) GS:ffff880c10e80000(0000)
  knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00000000a0965b00 CR3: 000000000200a003 CR4: 00000000007606e0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  PKRU: 55555554
  Call Trace:
   <IRQ>
   ib_poll_handler+0x43/0x80 [ib_core]
   irq_poll_softirq+0xae/0x110
   __do_softirq+0xd1/0x28c
   irq_exit+0xde/0xf0
   do_IRQ+0x54/0xe0
   common_interrupt+0x8f/0x8f
   </IRQ>
   ? cpuidle_enter_state+0xd9/0x2a0
   ? cpuidle_enter_state+0xc7/0x2a0
   ? do_idle+0x170/0x1d0
   ? cpu_startup_entry+0x6f/0x80
   ? start_secondary+0x1b9/0x210
   ? secondary_startup_64+0xa5/0xb0
  Code: 0f 87 e1 00 00 00 8b 4c 24 14 44 8b 43 14 89 c8 4d 63 c8 44 29 c0 99 31 d0 29 d0 31 d2 48 98 48 8d 04 80 48 8d 04 80 48 c1 e0 02 <49> f7 f1 48 83 f8 0a 0f 86 c1 00 00 00 44 39 c1 7f 10 48 89 df
  RIP: backport_rdma_dim+0x10e/0x240 [mlx_compat] RSP: ffff880c10e83ec0

Fixes: ef22e647db3c ("linux/dim: Implement RDMA adaptive moderation (DIM)")
Link: https://lore.kernel.org/r/20220627140004.3099-1-thomas.liu@ucloud.cn
Signed-off-by: Tao Liu <thomas.liu@ucloud.cn>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Acked-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>