mm: protect mremap() against SPF hanlder

If a thread is remapping an area while another one is faulting on the
destination area, the SPF handler may fetch the vma from the RB tree before
the pte has been moved by the other thread. This means that the moved ptes
will overwrite those create by the page fault handler leading to page
leaked.

	CPU 1				CPU2
	enter mremap()
	unmap the dest area
	copy_vma()			Enter speculative page fault handler
	   >> at this time the dest area is present in the RB tree
					fetch the vma matching dest area
					create a pte as the VMA matched
					Exit the SPF handler
					<data written in the new page>
	move_ptes()
	  > it is assumed that the dest area is empty,
 	  > the move ptes overwrite the page mapped by the CPU2.

To prevent that, when the VMA matching the dest area is extended or created
by copy_vma(), it should be marked as non available to the SPF handler.
The usual way to so is to rely on vm_write_begin()/end().
This is already in __vma_adjust() called by copy_vma() (through
vma_merge()). But __vma_adjust() is calling vm_write_end() before returning
which create a window for another thread.
This patch adds a new parameter to vma_merge() which is passed down to
vma_adjust().
The assumption is that copy_vma() is returning a vma which should be
released by calling vm_raw_write_end() by the callee once the ptes have
been moved.

Change-Id: Icd338ad6e9b3c97b7334d3b8d30a8badfa2a4efa
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Patch-mainline: linux-mm @ Tue, 17 Apr 2018 16:33:16
[vinmenon@codeaurora.org: changes in vma_merge arguments related
to the anon vma user name which is not suppported upstream.]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>