iommufd: Set end correctly when doing batch carry

Even though the test suite covers this it somehow became obscured that
this wasn't working.

The test iommufd_ioas.mock_domain.access_domain_destory would blow up
rarely.

end should be set to 1 because this just pushed an item, the carry, to the
pfns list.

Sometimes the test would blow up with:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] SMP
  CPU: 5 PID: 584 Comm: iommufd Not tainted 6.5.0-rc1-dirty #1236
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
  RIP: 0010:batch_unpin+0xa2/0x100 [iommufd]
  Code: 17 48 81 fe ff ff 07 00 77 70 48 8b 15 b7 be 97 e2 48 85 d2 74 14 48 8b 14 fa 48 85 d2 74 0b 40 0f b6 f6 48 c1 e6 04 48 01 f2 <48> 8b 3a 48 c1 e0 06 89 ca 48 89 de 48 83 e7 f0 48 01 c7 e8 96 dc
  RSP: 0018:ffffc90001677a58 EFLAGS: 00010246
  RAX: 00007f7e2646f000 RBX: 0000000000000000 RCX: 0000000000000001
  RDX: 0000000000000000 RSI: 00000000fefc4c8d RDI: 0000000000fefc4c
  RBP: ffffc90001677a80 R08: 0000000000000048 R09: 0000000000000200
  R10: 0000000000030b98 R11: ffffffff81f3bb40 R12: 0000000000000001
  R13: ffff888101f75800 R14: ffffc90001677ad0 R15: 00000000000001fe
  FS:  00007f9323679740(0000) GS:ffff8881ba540000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000000 CR3: 0000000105ede003 CR4: 00000000003706a0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
   <TASK>
   ? show_regs+0x5c/0x70
   ? __die+0x1f/0x60
   ? page_fault_oops+0x15d/0x440
   ? lock_release+0xbc/0x240
   ? exc_page_fault+0x4a4/0x970
   ? asm_exc_page_fault+0x27/0x30
   ? batch_unpin+0xa2/0x100 [iommufd]
   ? batch_unpin+0xba/0x100 [iommufd]
   __iopt_area_unfill_domain+0x198/0x430 [iommufd]
   ? __mutex_lock+0x8c/0xb80
   ? __mutex_lock+0x6aa/0xb80
   ? xa_erase+0x28/0x30
   ? iopt_table_remove_domain+0x162/0x320 [iommufd]
   ? lock_release+0xbc/0x240
   iopt_area_unfill_domain+0xd/0x10 [iommufd]
   iopt_table_remove_domain+0x195/0x320 [iommufd]
   iommufd_hw_pagetable_destroy+0xb3/0x110 [iommufd]
   iommufd_object_destroy_user+0x8e/0xf0 [iommufd]
   iommufd_device_detach+0xc5/0x140 [iommufd]
   iommufd_selftest_destroy+0x1f/0x70 [iommufd]
   iommufd_object_destroy_user+0x8e/0xf0 [iommufd]
   iommufd_destroy+0x3a/0x50 [iommufd]
   iommufd_fops_ioctl+0xfb/0x170 [iommufd]
   __x64_sys_ioctl+0x40d/0x9a0
   do_syscall_64+0x3c/0x80
   entry_SYSCALL_64_after_hwframe+0x46/0xb0

Link: https://lore.kernel.org/r/3-v1-85aacb2af554+bc-iommufd_syz3_jgg@nvidia.com
Cc: <stable@vger.kernel.org>
Fixes: f394576e ("iommufd: PFN handling for iopt_pages")
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Reported-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/iommu/iommufd/pages.c

@@ -297,7 +297,7 @@ static void batch_clear_carry(struct pfn_batch *batch, unsigned int keep_pfns)
 	batch->pfns[0] = batch->pfns[batch->end - 1] +
 			 (batch->npfns[batch->end - 1] - keep_pfns);
 	batch->npfns[0] = keep_pfns;
-	batch->end = 0;
+	batch->end = 1;
 }
 
 static void batch_skip_carry(struct pfn_batch *batch, unsigned int skip_pfns)