lib: fs: implement memory-mapped file i/o

lib/libfs/file.c

@@ -25,9 +25,22 @@ enum fs_status fs_file_read(
 	return status;
 }
 
-enum fs_status fs_file_write(
+enum fs_status fs_file_read_at(
 	struct fs_file *f,
 	struct xpc_buffer *buf,
+	off_t offset,
+	size_t count)
+{
+	if (!f->f_ops || !f->f_ops->f_read) {
+		return FS_ERR_NOT_IMPLEMENTED;
+	}
+
+	return f->f_ops->f_read(f, buf, count, &offset);
+}
+
+enum fs_status fs_file_write(
+	struct fs_file *f,
+	const struct xpc_buffer *buf,
 	size_t count)
 {
 	if (!f->f_ops || !f->f_ops->f_write) {