mm/huge_memory.c: fix potential NULL pointer dereference

mm/huge_memory.c: fix potential NULL pointer dereference

In collapse_huge_page() there is a race window between releasing the
mmap_sem read lock and taking the mmap_sem write lock, so find_vma() may
return NULL.  So check the return value to avoid NULL pointer dereference.

collapse_huge_page
	khugepaged_alloc_page
		up_read(&mm->mmap_sem)
	down_write(&mm->mmap_sem)
	vma = find_vma(mm, address)

Signed-off-by: Libin <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Wanpeng Li <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Cc: <[email protected]> # v3.0+
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>