fs/proc/task_mmu.c: assume non-NULL vma in pagemap_hugetlb()

fs/proc/task_mmu.c: assume non-NULL vma in pagemap_hugetlb()

Fengguang reported smatch error about potential NULL pointer access.

In updated page table walker, we never run ->hugetlb_entry() callback
on the address without vma. This is because __walk_page_range() checks
it in advance. So we can assume non-NULL vma in pagemap_hugetlb().

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>