arm64: mm: only perform memstart_addr sanity check if DEBUG_VM

Checking whether memstart_addr has been assigned every time it is
referenced adds a branch instruction that may hurt performance if
the reference in question occurs on a hot path. So only perform the
check if CONFIG_DEBUG_VM=y.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
[catalin.marinas@arm.com: replaced #ifdef with VM_BUG_ON]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/memory.h

@@ -132,9 +132,11 @@
 
 #ifndef __ASSEMBLY__
 
+#include <linux/mmdebug.h>
+
 extern phys_addr_t		memstart_addr;
 /* PHYS_OFFSET - the physical address of the start of memory. */
-#define PHYS_OFFSET		({ BUG_ON(memstart_addr & 1); memstart_addr; })
+#define PHYS_OFFSET		({ VM_BUG_ON(memstart_addr & 1); memstart_addr; })
 
 /* the offset between the kernel virtual and physical mappings */
 extern u64			kimage_voffset;