afs: Fix vlserver probe RTT handling

In the same spirit as commit ca57f022 ("afs: Fix fileserver probe
RTT handling"), don't rule out using a vlserver just because there
haven't been enough packets yet to calculate a real rtt.  Always set the
server's probe rtt from the estimate provided by rxrpc_kernel_get_srtt,
which is capped at 1 second.

This could lead to EDESTADDRREQ errors when accessing a cell for the
first time, even though the vl servers are known and have responded to a
probe.

Fixes: 1d4adfaf

 ("rxrpc: Make rxrpc_kernel_get_srtt() indicate validity")
Signed-off-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-afs@lists.infradead.org
Link: http://lists.infradead.org/pipermail/linux-afs/2023-June/006746.html
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>