mm: pagewalk: fix documentation of PTE hole handling

Empty PTEs are passed to the pte_entry callback, not to pte_hole.

Link: https://lkml.kernel.org/r/3695521.kQq0lBPeGt@devpool047


Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>