firmware: add call to LSM hook before firmware sysfs fallback

Add an LSM hook prior to allowing firmware sysfs fallback loading.

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>