when the entry of a DIR is put, write back.
A DIR entry should not be allocated a address_space

1. move pagetable, trapframe, kstack into mm_struct_t from proc_t, delete sz in proc_t.
2. modify exec, delete old mm of the proc and create a new mm.

Move the func which allocate clusters and update file size in disk to the eput, so when the file is close,
they access the disk.
This method increase the writing memory speed.