Convert g_mpu_region from single-CPU global variable to per-CPU array indexed
by this_cpu(). Change declaration from "unsigned int g_mpu_region" to
"unsigned int g_mpu_region[CONFIG_SMP_NCPUS]". Update all accesses in
mpu_allocregion(), mpu_freeregion(), and mpu_modify_region() to use
g_mpu_region[this_cpu()] for proper per-CPU MPU region tracking in multi-core
configurations.
Signed-off-by: hujun5 <hujun5@xiaomi.com>