vfs: Unmount /proc to release refcnts pertained to root

/proc must be unmounted to release refcnts which pertains to the root
mountpoint, i.e. zfs.
It was preventing zfs_umount from releasing the mp dentries properly,
thus VOP_INACTIVE from being called on the respective vnodes.

Found the problem while dumping the mountpoint refcnts.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

fs/vfs/main.cc

@@ -1592,6 +1592,12 @@ extern "C" void unmount_rootfs(void)
 
     sys_umount("/dev");
 
+    ret = sys_umount("/proc");
+    if (ret) {
+        kprintf("Warning: unmount_rootfs: failed to unmount /proc, "
+            "error = %s\n", strerror(ret));
+    }
+
     ret = sys_umount2("/", MNT_FORCE);
     if (ret) {
         kprintf("Warning: unmount_rootfs: failed to unmount /, "