afs: Fix fileserver rotation getting stuck

Fix the fileserver rotation code in a couple of ways:

 (1) op->server_states is an array, not a pointer to a single record, so
     fix the places that access it to index it.

 (2) In the places that go through an address list to work out which one
     has the best priority, fix the loops to skip known failed addresses.

Without this, the rotation algorithm may get stuck on addresses that are
inaccessible or don't respond.

This can be triggered manually by finding a server that advertises a
non-routable address and giving it a higher priority, eg.:

        echo "add udp 192.168.0.0/16 3000" >/proc/fs/afs/addr_prefs

if the server, say, includes the address 192.168.7.7 in its address list,
and then attempting to access a volume on that server.

Fixes: 495f2ae9

 ("afs: Fix fileserver rotation")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/4005300.1712309731@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/998836.1714746152@warthog.procyon.org.uk
Signed-off-by: Christian Brauner <brauner@kernel.org>