s390/pgtable: Make crdte() and cspg() return a value

Make the crdte() and cspg() wrappers return a boolean to indicate
success, like the other already existing "compare and swap" type of
wrappers.

Add documentation for those functions as well.

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>