drm: Don't free a struct never allocated by drm_gem_fb_init()

drm: Don't free a struct never allocated by drm_gem_fb_init()

drm_gem_fb_init() is passed the fb and never allocates it, so it should be
not the one freeing it. As it is now the second call to kfree() is possible
with the same fb. Coverity reported the following:

*** CID 1492613:  Memory - corruptions  (USE_AFTER_FREE)
/drivers/gpu/drm/drm_gem_framebuffer_helper.c: 230 in drm_gem_fb_create_with_funcs()
224     	fb = kzalloc(sizeof(*fb), GFP_KERNEL);
225     	if (!fb)
226     		return ERR_PTR(-ENOMEM);
227
228     	ret = drm_gem_fb_init_with_funcs(dev, fb, file, mode_cmd, funcs);
229     	if (ret) {
vvv     CID 1492613:  Memory - corruptions  (USE_AFTER_FREE)
vvv     Calling "kfree" frees pointer "fb" which has already been freed. [Note: The source code implementation of the function has been overridden by a user model.]
230     		kfree(fb);
231     		return ERR_PTR(ret);
232     	}
233
234     	return fb;
235     }

drm_gem_fb_init_with_funcs() calls drm_gem_fb_init()
drm_gem_fb_init() calls kfree(fb)

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1492613 ("Memory - corruptions")
Fixes: f2b816d78a94 ("drm/core: Allow drivers allocate a subclass of struct drm_framebuffer")
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415172024.24004-1-andrzej.p@collabora.com