arm64: ptdump: Don't override the level when operating on the stage-2 tables

arm64: ptdump: Don't override the level when operating on the stage-2 tables

Ptdump uses the init_mm structure directly to dump the kernel
pagetables. When ptdump is called on the stage-2 pagetables, this mm
argument is not used. Prevent the level from being overwritten by
checking the argument against NULL.

Signed-off-by: Sebastian Ene <sebastianene@google.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20240909124721.1672199-5-sebastianene@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>