riscv: Optimize kernel virtual address conversion macro

The current test in kernel_mapping_va_to_pa only applies when
CONFIG_XIP_KERNEL is set, so use IS_ENABLED to optimize this macro at
compile-time in standard kernels that do not require this test.
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Tested-by: Emil Renner Berthing <kernel@esmil.dk>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-By: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/page.h

@@ -122,7 +122,7 @@ extern struct kernel_mapping kernel_map;
 #define linear_mapping_va_to_pa(x)	((unsigned long)(x) - kernel_map.va_pa_offset)
 #define kernel_mapping_va_to_pa(y) ({						\
 	unsigned long _y = y;							\
-	(_y < kernel_map.virt_addr + XIP_OFFSET) ?					\
+	(IS_ENABLED(CONFIG_XIP_KERNEL) && _y < kernel_map.virt_addr + XIP_OFFSET) ?	\
 		((unsigned long)(_y) - kernel_map.va_kernel_xip_pa_offset) :		\
 		((unsigned long)(_y) - kernel_map.va_kernel_pa_offset - XIP_OFFSET);	\
 	})