afs: Don't use VL probe running state to make decisions outside probe code

Don't use the running state for VL server probes to make decisions about
which server to use as the state is cleared at the start of a probe and
intermediate values might also be misleading.

Instead, add a separate 'latest known' rtt in the afs_vlserver struct and a
flag to indicate if the server is known to be responding and update these
as and when we know what to change them to.

Fixes: 3bf0fb6f ("afs: Probe multiple fileservers simultaneously")
Signed-off-by: David Howells <dhowells@redhat.com>