kernel: file_t, fd_t and a few helper functions

kernel-src/include/kernel/file.h

@@ -0,0 +1,27 @@
+#ifndef FILE_H
+#define FILE_H
+
+#include <kernel/vfs.h>
+#include <mutex.h>
+
+typedef struct file_t {
+	vnode_t *vnode;
+	mutex_t mutex;
+	int refcount;
+	mode_t mode;
+	uintmax_t offset;
+	int flags;
+} file_t;
+
+typedef struct fd_t {
+	file_t *file;
+	int flags;
+} fd_t;
+
+file_t *fd_get(int fd);
+void fd_release(file_t *file);
+int fd_new(vnode_t *node, int flags, file_t **file, int *fd);
+int fd_close(int fd);
+int fd_dup(int oldfd, int newfd, bool exact, int flags, int *ret);
+
+#endif