NFS: remove set but not used variable 'mapping'

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>

fs/nfs/write.c

@@ -606,7 +606,6 @@ static void nfs_write_error(struct nfs_page *req, int error)
 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;
 
@@ -621,7 +620,6 @@ static int nfs_page_async_flush(struct nfs_pageio_descriptor *pgio,
 	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;