mm: prepare for FAULT_FLAG_SPECULATIVE

When speculating faults (without holding mmap_sem) we need to validate
that the vma against which we loaded pages is still valid when we're
ready to install the new PTE.

Therefore, replace the pte_offset_map_lock() calls that (re)take the
PTL with pte_map_lock() which can fail in case we find the VMA changed
since we started the fault.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[Port to 4.12 kernel]
[Remove the comment about the fault_env structure which has been
 implemented as the vm_fault structure in the kernel]
[move pte_map_lock()'s definition upper in the file]
[move the define of FAULT_FLAG_SPECULATIVE later in the series]
[review error path in do_swap_page(), do_anonymous_page() and
 wp_page_copy()]
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Change-Id: Id6dfae130fbfdd4bb92aa6415d6f1db7ef833266
[vinmenon@codeaurora.org: fix trivial merge conflicts]
Patch-mainline: linux-mm @ Tue, 17 Apr 2018 16:33:10
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>