x86/mm: Simplify the pgd_leaf() and p4d_leaf() checks a bit

The functions return bool, simplify the checks.

[ mingo: Split off from two other patches. ]

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250331081327.256412-6-bhe@redhat.com