xtensa: simdisk: add input size check in proc_write_simdisk

commit 5d5f08fd upstream.

A malicious user could pass an arbitrarily bad value
to memdup_user_nul(), potentially causing kernel crash.

This follows the same pattern as commit ee767463
("netdevsim: prevent bad user input in nsim_dev_health_break_write()")

Fixes: b6c7e873 ("xtensa: ISS: add host file-based simulated disk")
Fixes: 16e5c1fc ("convert a bunch of open-coded instances of memdup_user_nul()")
Cc: stable@vger.kernel.org
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Message-Id: <20250829083015.1992751-1-linmq006@gmail.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>