USB: class: CDC-ACM: fix race between get_serial and set_serial

USB: class: CDC-ACM: fix race between get_serial and set_serial

commit b41c1fa155ba56d125885b0191aabaf3c508d0a3 upstream.

TIOCGSERIAL is an ioctl. Thus it must be atomic. It returns
two values. Racing with set_serial it can return an inconsistent
result. The mutex must be taken.

In terms of logic the bug is as old as the driver. In terms of
code it goes back to the conversion to the get_serial and
set_serial methods.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@kernel.org>
Fixes: 99f75a1fcd865 ("cdc-acm: switch to ->[sg]et_serial()")
Link: https://lore.kernel.org/r/20240912141916.1044393-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>