io_uring: reexpand under-reexpanded iters

[   74.211232] BUG: KASAN: stack-out-of-bounds in iov_iter_revert+0x809/0x900
[   74.212778] Read of size 8 at addr ffff888025dc78b8 by task
syz-executor.0/828
[   74.214756] CPU: 0 PID: 828 Comm: syz-executor.0 Not tainted
5.14.0-rc3-next-20210730 #1
[   74.216525] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[   74.219033] Call Trace:
[   74.219683]  dump_stack_lvl+0x8b/0xb3
[   74.220706]  print_address_description.constprop.0+0x1f/0x140
[   74.224226]  kasan_report.cold+0x7f/0x11b
[   74.226085]  iov_iter_revert+0x809/0x900
[   74.227960]  io_write+0x57d/0xe40
[   74.232647]  io_issue_sqe+0x4da/0x6a80
[   74.242578]  __io_queue_sqe+0x1ac/0xe60
[   74.245358]  io_submit_sqes+0x3f6e/0x76a0
[   74.248207]  __do_sys_io_uring_enter+0x90c/0x1a20
[   74.257167]  do_syscall_64+0x3b/0x90
[   74.257984]  entry_SYSCALL_64_after_hwframe+0x44/0xae

old_size = iov_iter_count();
...
iov_iter_revert(old_size - iov_iter_count());

If iov_iter_revert() is done base on the initial size as above, and the
iter is truncated and not reexpanded in the middle, it miscalculates
borders causing problems. This trace is due to no one reexpanding after
generic_write_checks().

Now iters store how many bytes has been truncated, so reexpand them to
the initial state right before reverting.

Cc: stable@vger.kernel.org
Reported-by: Palash Oswal <oswalpalash@gmail.com>
Reported-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Reported-and-tested-by:  <syzbot+9671693590ef5aad8953@syzkaller.appspotmail.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>