fn check_user_mapping(vaddr: u64) -> Result<(), &'static str>Expand description
Checks that vaddr is mapped and USER_ACCESSIBLE at every level of the
page hierarchy (PML4 → PDPT → PD → PT).
Returns Ok(()) if all levels have PRESENT | USER_ACCESSIBLE set.
Otherwise returns Err(&str) naming the problematic level.
Handles 1 GiB (PDPT) and 2 MiB (PD) huge pages: stop descending once a huge-page entry is found with USER_ACCESSIBLE.