drm/i915/gvt: Use an "unsigned long" to iterate over memslot gfns

Use an "unsigned long" instead of an "int" when iterating over the gfns
in a memslot.  The number of pages in the memslot is tracked as an
"unsigned long", e.g. KVMGT could theoretically break if a KVM memslot
larger than 16TiB were deleted (2^32 * 4KiB).

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yongwei Ma <yongwei.ma@intel.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Link: https://lore.kernel.org/r/20230729013535.1070024-10-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>