arm64: Use simpler arithmetics for the linear map macros

Because of the tagged addresses, the __is_lm_address() and
__lm_to_phys() macros grew to some harder to understand bitwise
operations using PAGE_OFFSET. Since these macros only accept untagged
addresses, use a simple subtract operation.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20210201190634.22942-3-catalin.marinas@arm.com