Fixes gcc '-Wunused-but-set-variable' warning:
fs/nfs/write.c: In function nfs_page_async_flush:
fs/nfs/write.c:609:24: warning: variable mapping set but not used [-Wunused-but-set-variable]
It is not use since commit 
aefb623c422e ("NFS: Fix
writepage(s) error handling to not report errors twice")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
 
 static int nfs_page_async_flush(struct nfs_pageio_descriptor *pgio,
                                struct page *page)
 {
-       struct address_space *mapping;
        struct nfs_page *req;
        int ret = 0;
 
        WARN_ON_ONCE(test_bit(PG_CLEAN, &req->wb_flags));
 
        /* If there is a fatal error that covers this write, just exit */
-       mapping = page_file_mapping(page);
        ret = pgio->pg_error;
        if (nfs_error_is_fatal_on_server(ret))
                goto out_launder;