riscv: mm: init: mark satp_mode __ro_after_init

satp_mode is never modified after init, so it can be marked as
__ro_after_init.
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/mm/init.c

@@ -38,9 +38,9 @@ EXPORT_SYMBOL(kernel_map);
 #endif
 
 #ifdef CONFIG_64BIT
-u64 satp_mode = !IS_ENABLED(CONFIG_XIP_KERNEL) ? SATP_MODE_48 : SATP_MODE_39;
+u64 satp_mode __ro_after_init = !IS_ENABLED(CONFIG_XIP_KERNEL) ? SATP_MODE_48 : SATP_MODE_39;
 #else
-u64 satp_mode = SATP_MODE_32;
+u64 satp_mode __ro_after_init = SATP_MODE_32;
 #endif
 EXPORT_SYMBOL(satp_mode);