mm: Introduce lm_alias

Certain architectures may have the kernel image mapped separately to
alias the linear map. Introduce a macro lm_alias to translate a kernel
image symbol into its linear alias. This is used in part with work to
add CONFIG_DEBUG_VIRTUAL support for arm64.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

include/linux/mm.h

@@ -76,6 +76,10 @@ extern int mmap_rnd_compat_bits __read_mostly;
 #define page_to_virt(x)	__va(PFN_PHYS(page_to_pfn(x)))
 #endif
 
+#ifndef lm_alias
+#define lm_alias(x)	__va(__pa_symbol(x))
+#endif
+
 /*
  * To prevent common memory management code establishing
  * a zero page mapping on a read fault.