mm/huge_memory: make is_transparent_hugepage() static

It's only used inside the huge_memory.c now. Don't export it and make
it static. We can thus reduce the size of huge_memory.o a bit.

Without this patch:
   text	   data	    bss	    dec	    hex	filename
  32319	   2965	      4	  35288	   89d8	mm/huge_memory.o

With this patch:
   text	   data	    bss	    dec	    hex	filename
  32042	   2957	      4	  35003	   88bb	mm/huge_memory.o

Link: https://lkml.kernel.org/r/20220302082145.12028-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>