procfs: fix use-after-free

release_mp_dentries() releases those dentries, so they cannot be used
afterwards.  The vnode ought to be free at that point.

Reviewed-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>