media: don't drop front-end reference count for ->detach

A bugfix introduce a link failure in configurations without CONFIG_MODULES:

In file included from drivers/media/usb/dvb-usb/pctv452e.c:20:0:
drivers/media/usb/dvb-usb/pctv452e.c: In function 'pctv452e_frontend_attach':
drivers/media/dvb-frontends/stb0899_drv.h:151:36: error: weak declaration of 'stb0899_attach' being applied to a already existing, static definition

The problem is that the !IS_REACHABLE() declaration of stb0899_attach()
is a 'static inline' definition that clashes with the weak definition.

I further observed that the bugfix was only done for one of the five users
of stb0899_attach(), the other four still have the problem. This reverts
the bugfix and instead addresses the problem by not dropping the reference
count when calling '->detach()', instead we call this function directly
in dvb_frontend_put() before dropping the kref on the front-end.

I first submitted this in early 2018, and after some discussion it
was apparently discarded. While there is a long-term plan in place,
that plan is obviously not nearing completion yet, and the current
kernel is still broken unless this patch is applied.

Link: https://patchwork.kernel.org/patch/10140175/
Link: https://patchwork.linuxtv.org/patch/54831/

Cc: Max Kellermann <[e-mail]>
Cc: Wolfgang Rohdewald <[e-mail]>
Cc: [e-mail]
Fixes: f686c14364ad ("[media] stb0899: move code to "detach" callback")
Fixes: 6cdeaed3b142 ("media: dvb_usb_pctv452e: module refcount changes were unbalanced")
Signed-off-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Sean Young <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/dvb-core/dvb_frontend.c

···
 
 static void dvb_frontend_put(struct dvb_frontend *fe)
 {
+	/* call detach before dropping the reference count */
+	if (fe->ops.detach)
+		fe->ops.detach(fe);
 	/*
 	 * Check if the frontend was registered, as otherwise
 	 * kref was not initialized yet.
···
 	dvb_frontend_invoke_release(fe, fe->ops.release_sec);
 	dvb_frontend_invoke_release(fe, fe->ops.tuner_ops.release);
 	dvb_frontend_invoke_release(fe, fe->ops.analog_ops.release);
-	dvb_frontend_invoke_release(fe, fe->ops.detach);
 	dvb_frontend_put(fe);
 }
 EXPORT_SYMBOL(dvb_frontend_detach);

drivers/media/usb/dvb-usb/pctv452e.c

···
 						&a->dev->i2c_adap);
 	if (!a->fe_adap[0].fe)
 		return -ENODEV;
-
-	/*
-	 * dvb_frontend will call dvb_detach for both stb0899_detach
-	 * and stb0899_release but we only do dvb_attach(stb0899_attach).
-	 * Increment the module refcount instead.
-	 */
-	symbol_get(stb0899_attach);
-
 	if ((dvb_attach(lnbp22_attach, a->fe_adap[0].fe,
 					&a->dev->i2c_adap)) == NULL)
 		err("Cannot attach lnbp22\n");