s390/gup: reduce code duplication between [__]get_user_pages_fast functions

Just call __get_user_pages_fast() from get_user_pages_fast() like powerpc.
This saves a lot of duplicated code.

Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>