ALSA: core: Fix card races between register and disconnect

There is a small race window in the card disconnection code that
allows the registration of another card with the very same card id.
This leads to a warning in procfs creation as caught by syzkaller.

The problem is that we delete snd_cards and snd_cards_lock entries at
the very beginning of the disconnection procedure.  This makes the
slot available to be assigned for another card object while the
disconnection procedure is being processed.  Then it becomes possible
to issue a procfs registration with the existing file name although we
check the conflict beforehand.

The fix is simply to move the snd_cards and snd_cards_lock clearances
at the end of the disconnection procedure.  The references to these
entries are merely either from the global proc files like
/proc/asound/cards or from the card registration / disconnection, so
it should be fine to shift at the very end.

Reported-by:  <syzbot+48df349490c36f9f54ab@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>