Fix OID format for vm.swap_reserved and vm.swap_total

The correct OID format for CTLTYPE_U64 is QU (uquad_t), not A
(text expressed via char *).

This issue was noticed while doing an sysctl tree walk using a
sysctl(9) consumer that relies on the OID format to intuit what the
type should be for a given sysctl.

MFC after: 1 month
Sponsored by: DellEMC Isilon
Differential Revision: https://reviews.freebsd.org/D34877