mm/hugetlb.c: use huge_pte_lock instead of opencoding the lock

No functional change by this patch.

Link: http://lkml.kernel.org/r/20161018090234.22574-1-aneesh.kumar@linux.vnet.ibm.com


Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>