NFS: Fix an Oopsable condition in __nfs_pageio_add_request()

mainline inclusion
from mainline-v5.13-rc4
commit 56517ab9
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9BHTI
CVE: CVE-2021-47167

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=56517ab958b7c11030e626250c00b9b1a24b41eb



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Ensure that nfs_pageio_error_cleanup() resets the mirror array contents,
so that the structure reflects the fact that it is now empty.
Also change the test in nfs_pageio_do_add_request() to be more robust by
checking whether or not the list is empty rather than relying on the
value of pg_count.

Fixes: a7d42ddb ("nfs: add mirroring support to pgio layer")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Conflicts:
    fs/nfs/pagelist.c
Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>