Add TRAMP_SWAPPER_OFFSET and use that instead of hardcoding
the offset between swapper_pg_dir and tramp_pg_dir.

Then use TRAMP_SWAPPER_OFFSET to assert that the offset is
correct at link time.
Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20210202123658.22308-3-joey.gouly@arm.comSigned-off-by: Will Deacon <will@kernel.org>