[kernel] Allow passing 0 to vma_resize to query the current size

Passing a size of 0 in to vma_resize will now not attempt to alter the VMA size, but will
still put the size into the passed-in pointer. Using this allows querying the size of a
VMA without changing it.

definitions/objects/vma.def

@@ -32,6 +32,6 @@ object vma : object {
     }
 
     method resize [cap:resize] {
-        param size size [inout]
+        param size size [inout]  # New size for the VMA, or 0 to query the current size without changing
     }
 }