s390/mm: fix -Wunused-but-set-variable warnings

Convert two functions to static inline to get ride of W=1 GCC warnings
like,

mm/gup.c: In function 'gup_pte_range':
mm/gup.c:1816:16: warning: variable 'ptem' set but not used
[-Wunused-but-set-variable]
  pte_t *ptep, *ptem;
                ^~~~

mm/mmap.c: In function 'acct_stack_growth':
mm/mmap.c:2322:16: warning: variable 'new_start' set but not used
[-Wunused-but-set-variable]
  unsigned long new_start;
                ^~~~~~~~~

Signed-off-by: Qian Cai <cai@lca.pw>
Link: https://lore.kernel.org/lkml/1570138596-11913-1-git-send-email-cai@lca.pw/


Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>