fuse: fix the ->direct_IO() treatment of iov_iter

mainline inclusion
from mainline-5.9-rc7
commit 933a3752
category: bugfix
bugzilla: 42553
CVE: NA

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the callers rely upon having any iov_iter_truncate() done inside
->direct_IO() countered by iov_iter_reexpand().

Reported-by: Qian Cai <cai@redhat.com>
Tested-by: Qian Cai <cai@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Conflict: fs/fuse/file.c
commit 5da784cc("fuse: add max_pages to init_out") is not
backported, fuse_round_up only accept one paramater.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>