Impact: Replace and remove risky (non-EXPORTed) API

module_text_address() returns a pointer to the module, which given locking
improvements in module.c, is useless except to test for NULL:

1) If the module can't go away, use __module_text_address.
2) Otherwise, just use is_module_text_address().

Cc: linux-mtd@lists.infradead.org
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>