xen/gntdev.c: convert to use vm_map_pages()

xen/gntdev.c: convert to use vm_map_pages()

Convert to use vm_map_pages() to map range of kernel memory to user vma.

map->count is passed to vm_map_pages() and internal API verify map->count
against count ( count = vma_pages(vma)) for page array boundary overrun
condition.

Link: http://lkml.kernel.org/r/88e56e82d2db98705c2d842e9c9806c00b366d67.1552921225.git.jrdr.linux@gmail.com
Signed-off-by: Souptick Joarder <[email protected]>
Reviewed-by: Boris Ostrovsky <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Heiko Stuebner <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Kyungmin Park <[email protected]>
Cc: Marek Szyprowski <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Oleksandr Andrushchenko <[email protected]>
Cc: Pawel Osciak <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Robin Murphy <[email protected]>
Cc: Russell King <[email protected]>
Cc: Sandy Huang <[email protected]>
Cc: Stefan Richter <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Thierry Reding <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>