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>