include/linux/hugetlb.h: make isolate_huge_page() an inline

With CONFIG_HUGETLBFS=n:

  mm/migrate.c: In function `do_move_page_to_node_array':
  include/linux/hugetlb.h:140:33: warning: statement with no effect [-Wunused-value]
   #define isolate_huge_page(p, l) false
                                   ^
  mm/migrate.c:1170:4: note: in expansion of macro `isolate_huge_page'
      isolate_huge_page(page, &pagelist);

Reported-by: Borislav Petkov <bp@alien8.de>
Tested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>