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>

arch/s390/mm/gmap.c

@@ -2424,7 +2424,7 @@ EXPORT_SYMBOL_GPL(gmap_pmdp_idte_global);
  * This function is assumed to be called with the guest_table_lock
  * held.
  */
-bool gmap_test_and_clear_dirty_pmd(struct gmap *gmap, pmd_t *pmdp,
+static bool gmap_test_and_clear_dirty_pmd(struct gmap *gmap, pmd_t *pmdp,
 					  unsigned long gaddr)
 {
 	if (pmd_val(*pmdp) & _SEGMENT_ENTRY_INVALID)