mm/pagewalk.c: report holes in hugetlb ranges

mm/pagewalk.c: report holes in hugetlb ranges

This matters at least for the mincore syscall, which will otherwise copy
uninitialized memory from the page allocator to userspace.  It is
probably also a correctness error for /proc/$pid/pagemap, but I haven't
tested that.

Removing the `walk->hugetlb_entry` condition in walk_hugetlb_range() has
no effect because the caller already checks for that.

This only reports holes in hugetlb ranges to callers who have specified
a hugetlb_entry callback.

This issue was found using an AFL-based fuzzer.

v2:
 - don't crash on ->pte_hole==NULL (Andrew Morton)
 - add Cc stable (Andrew Morton)

Fixes: 1e25a271c8ac ("mincore: apply page table walker on do_mincore()")
Signed-off-by: Jann Horn <jannh@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Orabug: 27913118
CVE: CVE-2017-16994

(cherry picked from commit 373c4557d2aa362702c4c2d41288fb1e54990b7c)
Conflict: one line adjust due to huge_pte_offset() interface diff
Signed-off-by: Jack Vogel <jack.vogel@oracle.com>
Reviewed-by: Shan Hai <shan.hai@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>