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

lib/libfs/file.h

@@ -2,6 +2,7 @@
 #define _FS_FILE_H_
 
 #include "btree.h"
+#include "queue.h"
 
 #include <fs/dentry.h>
 #include <fs/file.h>
@@ -18,6 +19,8 @@ struct fs_file {
 	off_t f_seek;
 	struct fs_inode *f_inode;
 	struct fs_dentry *f_dent;
+
+	struct queue f_mappings;
 };
 
 #endif