mm: speculative page fault handler return VMA

When the speculative page fault handler is returning VM_RETRY, there is a
chance that VMA fetched without grabbing the mmap_sem can be reused by the
legacy page fault handler.  By reusing it, we avoid calling find_vma()
again. To achieve, that we must ensure that the VMA structure will not be
freed in our back. This is done by getting the reference on it (get_vma())
and by assuming that the caller will call the new service
can_reuse_spf_vma() once it has grabbed the mmap_sem.

can_reuse_spf_vma() is first checking that the VMA is still in the RB tree
, and then that the VMA's boundaries matched the passed address and release
the reference on the VMA so that it can be freed if needed.

In the case the VMA is freed, can_reuse_spf_vma() will have returned false
as the VMA is no more in the RB tree.

In the architecture page fault handler, the call to the new service
reuse_spf_or_find_vma() should be made in place of find_vma(), this will
handle the check on the spf_vma and if needed call find_vma().

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Change-Id: Ia56dcf807e8bddf6788fd696dd80372db35476f0
Patch-mainline: linux-mm @ Tue, 17 Apr 2018 16:33:28
[vinmenon@codeaurora.org: trivial merge conflicts]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>