um: pgtable.h: Fix W=1 warning for empty body in 'do' statement

Use the common kernel style to eliminate a warning:

./arch/um/include/asm/pgtable.h:305:47: warning: suggest braces around empty body in ‘do’ statement [-Wempty-body]
 #define update_mmu_cache(vma,address,ptep) do ; while (0)
                                               ^
mm/filemap.c:3212:3: note: in expansion of macro ‘update_mmu_cache’
   update_mmu_cache(vma, addr, vmf->pte);
   ^~~~~~~~~~~~~~~~

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: linux-um@lists.infradead.org
Signed-off-by: Richard Weinberger <richard@nod.at>