mm/hugetlb: avoid looping to the same hugepage if !pages and !vmas

When mmapping an existing hugetlbfs file with MAP_POPULATE, we find it
is very time consuming.  For example, mmapping a 128GB file takes about
50 milliseconds.  Sampling with perfevent shows it spends 99% time in
the same_page loop in follow_hugetlb_page().

samples: 205  of event 'cycles', Event count (approx.): 136686374
-  99.04%  test_mmap_huget  [kernel.kallsyms]  [k] follow_hugetlb_page
        follow_hugetlb_page
        __get_user_pages
        __mlock_vma_pages_range
        __mm_populate
        vm_mmap_pgoff
        sys_mmap_pgoff
        sys_mmap
        system_call_fastpath
        __mmap64

follow_hugetlb_page() is called with pages=NULL and vmas=NULL, so for
each hugepage, we run into the same_page loop for pages_per_huge_page()
times, but doing nothing.  With this change, it takes less then 1
millisecond to mmap a 128GB file in hugetlbfs.

Link: http://lkml.kernel.org/r/1567581712-5992-1-git-send-email-totty.lu@gmail.com
Signed-off-by: Zhigang Lu <tonnylu@tencent.com>
Reviewed-by: Haozhong Zhang <hzhongzhang@tencent.com>
Reviewed-by: Zongming Zhang <knightzhang@tencent.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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