UPSTREAM: mm: Introduce lm_alias

(cherry-pick from commit 568c5fe5a54f2654f5a4c599c45b8a62ed9a2013)

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>
Bug: 20045882
Change-Id: I93ba4ef8df77f7e52717ab6e208d06122ac3a72b