staging: ramster: decrease foregin pers pages when count < 0

commit 9a5c5968 ("staging: ramster: Provide accessory functions for
counter decrease") forget decrease foregin pers pages, this patch fix
it.
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zcache/ramster/ramster.c

@@ -508,6 +508,7 @@ void ramster_count_foreign_pages(bool eph, int count)
 		if (count > 0) {
 			inc_ramster_foreign_pers_pages();
 		} else {
+			dec_ramster_foreign_pers_pages();
 			WARN_ON_ONCE(ramster_foreign_pers_pages < 0);
 		}
 	}