ALSA: seq: Avoid client data changes during proc reads

The proc read of each client should protect against the concurrent
data changes to keep the data consistent; although they are supposed
to be safe and won't crash things, it doesn't guarantee the
consistency between the read values.  Take client->ioctl_mutex for
protecting against the concurrent changes.

Link: https://patch.msgid.link/20250307084246.29271-2-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>