s390/mm: avoid using pmd_to_page for !USE_SPLIT_PMD_PTLOCKS

pmd_to_page() is only available if USE_SPLIT_PMD_PTLOCKS is defined.
The use of pmd_to_page in the gmap code can cause compile errors if
NR_CPUS is smaller than SPLIT_PTLOCK_CPUS. Do not use pmd_to_page
outside of USE_SPLIT_PMD_PTLOCKS sections.

Reported-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>