memory: fix debug allocator interaction with reclaimer

malloc() must wait for memory, and since page table operations can
allocate memory, it must be able to dip into the reserve pool.  free()
should indicate it is a reclaimer.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>