xtensa: simdisk: add input size check in proc_write_simdisk

In the Linux kernel, the following vulnerability has been resolved: xtensa: simdisk: add input size check in proc_write_simdisk A malicious user could pass an arbitrarily bad value to memdup_user_nul(), potentially causing kernel crash. This follows the same pattern as commit ee76746387f6 ("netdevsim: prevent bad user input in nsim_dev_health_break_write()")