iommufd: Do not corrupt the pfn list when doing batch carry

In the Linux kernel, the following vulnerability has been resolved:

iommufd: Do not corrupt the pfn list when doing batch carry

If batch->end is 0 then setting npfns[0] before computing the new value of
pfns will fail to adjust the pfn and result in various page accounting
corruptions. It should be ordered after.

This seems to result in various kinds of page meta-data corruption related
failures:

WARNING: CPU: 1 PID: 527 at mm/gup.c:75 try_grab_folio+0x503/0x740
Modules linked in:
CPU: 1 PID: 527 Comm: repro Not tainted 6.3.0-rc2-eeac8ede1755+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
RIP: 0010:try_grab_folio+0x503/0x740
Code: e3 01 48 89 de e8 6d c1 dd ff 48 85 db 0f 84 7c fe ff ff e8 4f bf dd ff 49 8d 47 ff 48 89 45 d0 e9 73 fe ff ff e8 3d bf dd ff <0f> 0b 31 db e9 d0 fc ff ff e8 2f bf dd ff 48 8b 5d c8 31 ff 48 89
RSP: 0018:ffffc90000f37908 EFLAGS: 00010046
RAX: 0000000000000000 RBX: 00000000fffffc02 RCX: ffffffff81504c26
RDX: 0000000000000000 RSI: ffff88800d030000 RDI: 0000000000000002
RBP: ffffc90000f37948 R08: 000000000003ca24 R09: 0000000000000008
R10: 000000000003ca00 R11: 0000000000000023 R12: ffffea000035d540
R13: 0000000000000001 R14: 0000000000000000 R15: ffffea000035d540
FS: 00007fecbf659740(0000) GS:ffff88807dd00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000200011c3 CR3: 000000000ef66006 CR4: 0000000000770ee0
PKRU: 55555554
Call Trace:
<TASK>
internal_get_user_pages_fast+0xd32/0x2200
pin_user_pages_fast+0x65/0x90
pfn_reader_user_pin+0x376/0x390
pfn_reader_next+0x14a/0x7b0
pfn_reader_first+0x140/0x1b0
iopt_area_fill_domain+0x74/0x210
iopt_table_add_domain+0x30e/0x6e0
iommufd_device_selftest_attach+0x7f/0x140
iommufd_test+0x10ff/0x16f0
iommufd_fops_ioctl+0x206/0x330
__x64_sys_ioctl+0x10e/0x160
do_syscall_64+0x3b/0x90
entry_SYSCALL_64_after_hwframe+0x72/0xdc