powerpc/book3s64: fix dump_linuxpagetables "present" flag

Since commit bd0dbb73 ("powerpc/mm/books3s: Add new pte bit to
mark pte temporarily invalid."), _PAGE_PRESENT doesn't mean exactly
that a page is present. A page is also considered preset when
_PAGE_INVALID is set.

This patch changes the meaning of "present" and adds a status "valid"
associated to the _PAGE_PRESENT flag.

Fixes: bd0dbb73 ("powerpc/mm/books3s: Add new pte bit to mark pte temporarily invalid.")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>