fuse: don't ignore errors from fuse_writepages_fill()

mainline inclusion
from mainline-v5.8-rc6
commit 7779b047
category: bugfix
bugzilla: 39163
CVE: NA

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fuse_writepages() ignores some errors taken from fuse_writepages_fill() I
believe it is a bug: if .writepages is called with WB_SYNC_ALL it should
either guarantee that all data was successfully saved or return error.

Fixes: 26d614df ("fuse: Implement writepages callback")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>