NFS: Don't corrupt the value of pg_bytes_written in nfs_do_recoalesce()

stable inclusion
from linux-4.19.193
commit 40f139a6d50c232c0d1fd1c5e65a845c62db0ede

--------------------------------

commit 0d0ea309 upstream.

The value of mirror->pg_bytes_written should only be updated after a
successful attempt to flush out the requests on the list.

Fixes: a7d42ddb ("nfs: add mirroring support to pgio layer")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>