s390/mm: better annotate 2KB pagetable fragments handling

Explicitly encode immediate value of pending remove nibble
(bits 31-28) and tracking nibble (bits 27-24) of the page
refcount whenever these nibbles are tested or changed, for
better readability. Also, add some comments describing how
the fragments are handled.

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