how to pass pointer from user to kernel in xv6

the addr in usermode and kernel mode it different. so while passing pointer to kernel mode, we should just pass a uint64 var to it.
then we can get it through function as a var.

it is more like a offset, later we can locate it.

argaddr works same like argint, it capture the arg of syscall from proc sturcture

then we can get the real address of the virtual address by its pagetable and offset easily

memmove part is pretty simple, kernel mode has the right to access all address