IB/hfi1: Restore kfree in dummy_netdev cleanup

We need to do some rework on the dummy netdev. Calling the free_netdev()
would normally make sense, and that will be addressed in an upcoming
patch. For now just revert the behavior to what it was before keeping the
unused variable removal part of the patch.

The dd->dumm_netdev is mainly used for packet receiving through
alloc_netdev_mqs() for typical net devices. A a result, it should be freed
with kfree instead of free_netdev() that leads to a crash when unloading
the hfi1 module:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 8000000855b54067 P4D 8000000855b54067 PUD 84a4f5067 PMD 0
  Oops: 0000 [#1] SMP PTI
  CPU: 73 PID: 10299 Comm: modprobe Not tainted 5.6.0-rc5+ #1
  Hardware name: Intel Corporation S2600WT2R/S2600WT2R, BIOS SE5C610.86B.01.01.0016.033120161139 03/31/2016
  RIP: 0010:__hw_addr_flush+0x12/0x80
  Code: 40 00 48 83 c4 08 4c 89 e7 5b 5d 41 5c e9 76 77 18 00 66 0f 1f 44 00 00 0f 1f 44 00 00 41 54 49 89 fc 55 53 48 8b 1f 48 39 df <48> 8b 2b 75 08 eb 4a 48 89 eb 48 89 c5 48 89 df e8 99 bf d0 ff 84
  RSP: 0018:ffffb40e08783db8 EFLAGS: 00010282
  RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000002
  RDX: ffffb40e00000000 RSI: 0000000000000246 RDI: ffff88ab13662298
  RBP: ffff88ab13662000 R08: 0000000000001549 R09: 0000000000001549
  R10: 0000000000000001 R11: 0000000000aaaaaa R12: ffff88ab13662298
  R13: ffff88ab1b259e20 R14: ffff88ab1b259e42 R15: 0000000000000000
  FS:  00007fb39b534740(0000) GS:ffff88b31f940000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000000 CR3: 000000084d3ea004 CR4: 00000000003606e0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
   dev_addr_flush+0x15/0x30
   free_netdev+0x7e/0x130
   hfi1_netdev_free+0x59/0x70 [hfi1]
   remove_one+0x65/0x110 [hfi1]
   pci_device_remove+0x3b/0xc0
   device_release_driver_internal+0xec/0x1b0
   driver_detach+0x46/0x90
   bus_remove_driver+0x58/0xd0
   pci_unregister_driver+0x26/0xa0
   hfi1_mod_cleanup+0xc/0xd54 [hfi1]
   __x64_sys_delete_module+0x16c/0x260
   ? exit_to_usermode_loop+0xa4/0xc0
   do_syscall_64+0x5b/0x200
   entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: 193ba03141 ("IB/hfi1: Use free_netdev() in hfi1_netdev_free()")
Link: https://lore.kernel.org/r/20200623203224.106975.16926.stgit@awfm-01.aw.intel.com
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/hfi1/netdev_rx.c

@@ -373,7 +373,7 @@ void hfi1_netdev_free(struct hfi1_devdata *dd)
 {
 	if (dd->dummy_netdev) {
 		dd_dev_info(dd, "hfi1 netdev freed\n");
-		free_netdev(dd->dummy_netdev);
+		kfree(dd->dummy_netdev);
 		dd->dummy_netdev = NULL;
 	}
 }