debugfs: Make debugfs_allow RO after init

Since debugfs_allow is only set at boot time during __init, make it
read-only after being set.

Fixes: a24c6f7b ("debugfs: Add access restriction option")
Cc: Peter Enderborg <peter.enderborg@sony.com>
Reviewed-by: Peter Enderborg <peter.enderborg@sony.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210405213959.3079432-1-keescook@chromium.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>