mm: introduce pte_spinlock for FAULT_FLAG_SPECULATIVE

When handling page fault without holding the mmap_sem the fetch of the
pte lock pointer and the locking will have to be done while ensuring
that the VMA is not touched in our back.

So move the fetch and locking operations in a dedicated function.

Change-Id: If93ab95b1d22b7195e1c15b57315021f6be7c394
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Patch-mainline: linux-mm @ Tue, 17 Apr 2018 16:33:11
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>