s390/mm: make gmap_test_and_clear_dirty_pmd static

Since gmap_test_and_clear_dirty_pmd is not exported and has no reason to
be globally visible make it static to avoid the following sparse warning:
arch/s390/mm/gmap.c:2427:6: warning: symbol 'gmap_test_and_clear_dirty_pmd' was not declared. Should it be static?

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>