arm64: Fix kernel address detection of __is_lm_address()

commit 519ea6f1 upstream.

Currently, the __is_lm_address() check just masks out the top 12 bits
of the address, but if they are 0, it still yields a true result.
This has as a side effect that virt_addr_valid() returns true even for
invalid virtual addresses (e.g. 0x0).

Fix the detection checking that it's actually a kernel address starting
at PAGE_OFFSET.

Fixes: 68dd8ef3

 ("arm64: memory: Fix virt_addr_valid() using __is_lm_address()")
Cc: <stable@vger.kernel.org> # 5.4.x
Cc: Will Deacon <will@kernel.org>
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20210126134056.45747-1-vincenzo.frascino@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>