usb: cdc-acm: Fix handling of oversized fragments

commit 12e712964f41d05ae034989892de445781c46730 upstream.

If we receive an initial fragment of size 8 bytes which specifies a wLength
of 1 byte (so the reassembled message is supposed to be 9 bytes long), and
we then receive a second fragment of size 9 bytes (which is not supposed to
happen), we currently wrongly bypass the fragment reassembly code but still
pass the pointer to the acm->notification_buffer to
acm_process_notification().

Make this less wrong by always going through fragment reassembly when we
expect more fragments.

Before this patch, receiving an overlong fragment could lead to `newctrl`
in acm_process_notification() being uninitialized data (instead of data
coming from the device).

Cc: stable <stable@kernel.org>
Fixes: ea258352 ("cdc-acm: reassemble fragmented notifications")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>