mem: Fix zero_page_pfn type

It is compared to u64, so should be such as well
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/kerndat.h

@@ -19,6 +19,6 @@ extern int tcp_max_wshare;
 extern int tcp_max_rshare;
 
 extern int kern_last_cap;
-extern unsigned long zero_page_pfn;
+extern u64 zero_page_pfn;
 
 #endif /* __CR_KERNDAT_H__ */

kerndat.c

@@ -154,7 +154,7 @@ out:
 }
 
 /* The page frame number (PFN) is constant for the zero page */
-unsigned long zero_page_pfn;
+u64 zero_page_pfn;
 
 static int init_zero_page_pfn()
 {