mm: don't do swap readahead during speculative page fault

Vinayak Menon faced a panic because one thread was page faulting a page in
swap, while another one was mprotecting a part of the VMA leading to a VMA
split.
This raise a panic in swap_vma_readahead() because the VMA's boundaries
were not more matching the faulting address.

To avoid this, if the page is not found in the swap, the speculative page
fault is aborted to retry a regular page fault.

Change-Id: Ia9d99fb5fde7bd89f38966838d115b6c8c15c9db
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Patch-mainline: linux-mm @ Mon, 5 Nov 2018 18:43:01 +0100
[vinmenon@codeaurora.org: a minor 80+ line fix]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>