Add free_sector

kernel/malloc.asm

@@ -128,6 +128,7 @@ free:
   ret
 
 ; Allocate a sector, 512 bytes or 32 paragraphs
+; OUT BX ptr to sector
 malloc_sector:
   call malloc_get_table
   mov cx, 0x0200
@@ -152,6 +153,16 @@ malloc_sector:
   xchg bx, di
   ret
 
+; Free a sector
+; IN BX ptr to sector
+free_sector:
+  call malloc_calc_offsets
+  xor ax, ax
+  add di, bx
+  stosw
+  stosw
+  ret
+
 fatal_oom:
   call printf
   db "PANIC OOM", 0x0A, 0x0D, 0x00