mmu: give the jvm balloon write permissions

Up until now the JVM balloon explicitly forbade writes to its range, because
we didn't expect the JVM to ever write to it. But with the recently problem
Gleb discovered of double-copying of objects inside the Heap, we will use
writes to figure out when that object is no longer a balloon. But still,
we need to be able to go back to the JVM specific fault handler for that.

Therefore, we need write permissions in the VMA itself.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
[ penberg: use perm_rw as suggested by gleb ]
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>