The description of file deletion in rust-fatfs is rather vague. You
can't just delete files from the directory, but you need to truncate the
file first, then delete the file, and finally update the metadata
information.

when task is over, we need delete file if it had been masked unlink.

1. Fix error return value of file system call
2. Do not run mapped to address 0
3. When a task copies data from user mode, the virtual address needs to
   be valid first.

1.fix bugs about timer interrupt
2.fix bug about huge map
3.fix bug about write

1. Increase user stack
2. When the thread exits, the memory is reclaimed in advance.

1. fix bugs about float number
2. fix bugs about user stack size
3. fix bugs about mmap
4. fix bugs about user's argument

net is so hard

update copy_to_user/copy_from_user impl

Dealing with tty echo issues.

add more syscall

make \r -> \n when stdin

add more syscall to support busybox

We need to ensure that the status update and reference count reduction
of the task exit are atomic.
When the task exits, the waiting task needs to be woken up according to
the value of clear_child_tid.

Now we use bitmap instead of buddy.
Buddy's implementation may contain bugs.