mm: protect VMA modifications using VMA sequence count

The VMA sequence count has been introduced to allow fast detection of
VMA modification when running a page fault handler without holding
the mmap_sem.

This patch provides protection against the VMA modification done in :
	- madvise()
	- mpol_rebind_policy()
	- vma_replace_policy()
	- change_prot_numa()
	- mlock(), munlock()
	- mprotect()
	- mmap_region()
	- collapse_huge_page()
	- userfaultd registering services

In addition, VMA fields which will be read during the speculative fault
path needs to be written using WRITE_ONCE to prevent write to be split
and intermediate values to be pushed to other CPUs.

Change-Id: Ic36046b7254e538b6baf7144c50ae577ee7f2074
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Patch-mainline: linux-mm @ Tue, 17 Apr 2018 16:33:15
[vinmenon@codeaurora.org: trivial merge conflict fixes]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
[charante@codeaurora.org: trivial merge conflict fixes]
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>