extmod/vfs: Use MP_REGISTER_ROOT_POINTER().

This uses MP_REGISTER_ROOT_POINTER() to register vfs_cur and
vfs_mount_table instead of using a conditional inside of mp_state_vm_t.

Signed-off-by: David Lechner <david@pybricks.com>

extmod/vfs.c

@@ -547,4 +547,7 @@ int mp_vfs_mount_and_chdir_protected(mp_obj_t bdev, mp_obj_t mount_point) {
     return ret;
 }
 
+MP_REGISTER_ROOT_POINTER(struct _mp_vfs_mount_t *vfs_cur);
+MP_REGISTER_ROOT_POINTER(struct _mp_vfs_mount_t *vfs_mount_table);
+
 #endif // MICROPY_VFS

py/mpstate.h

@@ -198,11 +198,6 @@ typedef struct _mp_state_vm_t {
 
     // root pointers for extmod
 
-    #if MICROPY_VFS
-    struct _mp_vfs_mount_t *vfs_cur;
-    struct _mp_vfs_mount_t *vfs_mount_table;
-    #endif
-
     #if MICROPY_PY_BLUETOOTH
     mp_obj_t bluetooth;
     #endif