Fix loading large process images.

2MiB large pages were being used for any large page mapping, but the
page manager doesn't correctly handle them everywhere yet. Now only
allow them for offset pointers (eg MMIO space) that will never be
unmapped.

src/drivers/nulldrv/main.s

@@ -1,15 +1,22 @@
+section .bss
+mypid: resq 1
+mychild: resq 1
+
+section .text
 global _start
 _start:
 	xor rbp, rbp	; Sentinel rbp
 
 	mov rax, 5		; GETPID syscall
 	int 0xee
-	mov r12, rax	; save pid to r12
+	mov [mypid], rax
 
 	mov rax, 8		; FORK syscall
 	int 0xee
+	mov [mychild], rax
 
-	mov r10, rax
+	mov r12, [mypid]
+	mov r13, [mychild]
 	mov rax, 1		; DEBUG syscall
 	int 0xee
 
@@ -22,7 +29,7 @@ _start:
 	mov rbx, 20		; sleep timeout
 
 .loop:
-	mov rax, 2		; MESSAGE syscall
+	mov rax, 1		; MESSAGE syscall
 	;mov rax, 0		; NOOP syscall
 	;syscall
 	int 0xee