vswitchd: Fix 100% CPU usage with bonds and --fake-proc-net.

The current date in milliseconds since the epoch is ~1,282,841,552,000,
which is greater than LONG_MAX of 4,294,967,295 on 32-bit systems, so
no matter what was stored into bond_next_fake_iface_update, it would always
appear to be expired.  It really needs to be a 64-bit number.  (This was
just a typo really.)

Since XenServer 5.5 requires --fake-proc-net, this probably fixes an
important bug there.

Reported-by: Luiz Henrique Ozaki <luiz.ozaki@gmail.com>