[SPARC]: Do not BUG() in srmmu_pte_pfn().

For device memory, just return a value that will never
cause pte_pfn() to return true.

Noticed by Tom Callaway.
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc/mm/srmmu.c

@@ -133,11 +133,12 @@ static struct bit_map srmmu_nocache_map;
 static unsigned long srmmu_pte_pfn(pte_t pte)
 {
 	if (srmmu_device_memory(pte_val(pte))) {
-		/* XXX Anton obviously had something in mind when he did this.
-		 * But what?
+		/* Just return something that will cause
+		 * pfn_valid() to return false.  This makes
+		 * copy_one_pte() to just directly copy to
+		 * PTE over.
 		 */
-		/* return (struct page *)~0; */
-		BUG();
+		return ~0UL;
 	}
 	return (pte_val(pte) & SRMMU_PTE_PMASK) >> (PAGE_SHIFT-4);
 }