Add mmap module in vm defining mmap_entry structure and some helper functions

src/vm/mmap.c

@@ -0,0 +1 @@
+#include "mmap.h"

src/vm/mmap.h

@@ -0,0 +1,18 @@
+#ifndef VM_MMAP_H
+#define VM_MMAP_H
+
+#include <hash.h>
+
+/* A mapping identifier type. */
+typedef unsigned mapid_t;
+
+/* A structure to represent a memory mapped file. */
+struct mmap_entry {
+  mapid_t mapping;      /* The mapping identifier of the mapped file. */
+  struct file *file;    /* A pointer to the file that is being mapped. */
+  void *upage;          /* The start address of the file data in the user VM. */
+
+  struct hash_elem elem; /* An elem for the hash table. */
+};
+
+#endif /* vm/mmap.h */