xen/grant: Fix compile warning.

drivers/xen/grant-table.c:85: warning: ‘rc’ may be used uninitialized in this function
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/grant-table.c

@@ -82,7 +82,7 @@ static inline grant_ref_t *__gnttab_entry(grant_ref_t entry)
 static int get_free_entries(unsigned count)
 {
 	unsigned long flags;
-	int ref, rc;
+	int ref, rc = 0;
 	grant_ref_t head;
 
 	spin_lock_irqsave(&gnttab_list_lock, flags);