openpromfs: finish conversion to the new mount API

The original mount API conversion inexplicably left out the change
from ->remount_fs to ->reconfigure; do that now.

Fixes: 7ab2fa76

 ("vfs: Convert openpromfs to use the new mount API")
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Link: https://lore.kernel.org/r/90b968aa-c979-420f-ba37-5acc3391b28f@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>