mm: add speculative page fault vmstats

Add speculative_pgfault vmstat counter to count successful speculative page
fault handling.

Also fixing a minor typo in include/linux/vm_event_item.h.

Change-Id: I0d3f3dc5195e1156d4b8edf83aff9d8d85904e8e
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Patch-mainline: linux-mm @ Tue, 17 Apr 2018 16:33:29
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>