ALSA: seq: Fix nested rwsem annotation for lockdep splat

syzkaller reported the lockdep splat due to the possible deadlock of
grp->list_mutex of each sequencer client object.  Actually this is
rather a false-positive report due to the missing nested lock
annotations.  The sequencer client may deliver the event directly to
another client which takes another own lock.

For addressing this issue, this patch replaces the simple down_read()
with down_read_nested().  As a lock subclass, the already existing
"hop" can be re-used, which indicates the depth of the call.

Reference: http://lkml.kernel.org/r/089e082686ac9b482e055c832617@google.com
Reported-by: syzbot <bot+7feb8de6b4d6bf810cf098bef942cc387e79d0ad@syzkaller.appspotmail.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>