gnttab: fix out of range shift count

Commit 213f145114 ("gnttab: fix/adjust gnttab_transfer()") wasn't
careful enough in this regard.

Coverity ID: 1306859
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -1726,7 +1726,8 @@ gnttab_transfer(
         max_bitsize = domain_clamp_alloc_bitsize(
             e, e->grant_table->gt_version > 1 || paging_mode_translate(e)
                ? BITS_PER_LONG + PAGE_SHIFT : 32 + PAGE_SHIFT);
-        if ( (1UL << (max_bitsize - PAGE_SHIFT)) <= mfn )
+        if ( max_bitsize < BITS_PER_LONG + PAGE_SHIFT &&
+             (mfn >> (max_bitsize - PAGE_SHIFT)) )
         {
             struct page_info *new_page;