xen: change the limit of nr_extents to UINT_MAX >> MEMOP_EXTENT_SHIFT

xen: change the limit of nr_extents to UINT_MAX >> MEMOP_EXTENT_SHIFT

Currently do_memory_op has a different maximum limit for nr_extents on
32 bit and 64 bit.
Change the limit to UINT_MAX >> MEMOP_EXTENT_SHIFT, so that it is the
same in both cases.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Committed-by: Ian Campbell <ian.campbell@citrix.com>