xen/gntdev,gntalloc: Remove unneeded VM flags

The only time when granted pages need to be treated specially is when
using Xen's PTE modification for grant mappings owned by another domain
(that is, only gntdev on PV guests).  Otherwise, the area does not
require VM_DONTCOPY and VM_PFNMAP, since it can be accessed just like
any other page of RAM.

Since the vm_operations_struct close operations decrement reference
counts, a corresponding open function that increments them is required
now that it is possible to have multiple references to a single area.

We are careful in the gntdev to check if we can remove those flags. The
reason that we need to be careful in gntdev on PV guests is because we are
not changing the PFN/MFN mapping on PV; instead, we change the application's
page tables to point to the other domain's memory. This means that the vma
cannot be copied without using another grant mapping hypercall; it also
requires special handling on unmap, which is the reason for gntdev's
dependency on the MMU notifier.

For gntalloc, this is not a concern - the pages are owned by the domain
using the gntalloc device, and can be mapped and unmapped in the same manner
as any other page of memory.

Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[v2: Added in git commit "We are.." from email correspondence]