s390/mm: use full 4KB page for 2KB PTE

Cease using 4KB pages to host two 2KB PTEs. That greatly
simplifies the memory management code at the expense of
page tables memory footprint.

Instead of two PTEs per 4KB page use only upper half of
the parent page for a single PTE. With that the list of
half-used pages pgtable_list becomes unneeded.

Further, the upper byte of the parent page _refcount
counter does not need to be used for fragments tracking
and could be left alone.

Commit 8211dad6 ("s390: add pte_free_defer() for
pgtables sharing page") introduced the use of PageActive
flag to coordinate a deferred free with 2KB page table
fragments tracking. Since there is no tracking anymore,
there is no need for using PageActive flag.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>