fs-randomfail: Operation is unfinished on EAGAIN, not ENOENT.

This caused crashes and other problems when using randomfail with
asynchronous fs drivers.

src/lib-fs/fs-randomfail.c

@@ -259,7 +259,7 @@ static int
 fs_file_random_fail_end(struct randomfail_fs_file *file,
 			int ret, enum fs_op op)
 {
-	if (ret == 0 || errno != ENOENT) {
+	if (ret == 0 || errno != EAGAIN) {
 		if (fs_random_fail(file->file.fs, 2, op))
 			return -1;
 		file->op_pending[op] = FALSE;