fbcon: Fix delayed takeover lockingdrm-misc-next-2022-04-14

I messed up the delayed takover path in the locking conversion in
6e7da3af008b ("fbcon: Move console_lock for register/unlink/unregister").

If CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER is enabled, fbcon take-over
doesn't take place when calling fbcon_fb_registered(). Instead, is deferred
using a workqueue and its fbcon_register_existing_fbs() function calls to
fbcon_fb_registered() again for each registered fbcon fb.

This leads to the console_lock tried to be held twice, causing a deadlock.

Fix it by re-extracting the lockless function and using it in the
delayed takeover path, where we need to hold the lock already to
iterate over the list of already registered fb. Well the current code
still is broken in there (since the list is protected by a
registration_lock, which we can't take here because it nests the other
way round with console_lock), but in the future this will be a list
protected by console_lock when this is all sorted out.

While reviewing the broken commit I realized that I've left some
outdated comments about the locking behind. Fix those too.

v2: Improve commit message (Javier)

Reported-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Fixes: 6e7da3af008b ("fbcon: Move console_lock for register/unlink/unregister")
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Du Cheng <ducheng2@gmail.com>
Cc: Claudio Suarez <cssk@net-c.es>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Zheyu Ma <zheyuma97@gmail.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Helge Deller <deller@gmx.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220413082128.348186-1-daniel.vetter@ffwll.ch

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