NFS: Ensure nfs_readpage returns promptly when internal error occurs

A previous refactoring of nfs_readpage() might end up calling
wait_on_page_locked_killable() even if readpage_async_filler() failed
with an internal error and pg_error was non-zero (for example, if
nfs_create_request() failed).  In the case of an internal error,
skip over wait_on_page_locked_killable() as this is only needed
when the read is sent and an error occurs during completion handling.
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/read.c

@@ -373,10 +373,10 @@ int nfs_readpage(struct file *file, struct page *page)
 			     &nfs_async_read_completion_ops);
 
 	ret = readpage_async_filler(&desc, page);
+	if (ret)
+		goto out;
 
-	if (!ret)
 	nfs_pageio_complete_read(&desc.pgio);
-
 	ret = desc.pgio.pg_error < 0 ? desc.pgio.pg_error : 0;
 	if (!ret) {
 		ret = wait_on_page_locked_killable(page);