afs: Fix missing lock when replacing VL server list

When afs_update_cell() replaces the cell->vl_servers list, it uses RCU
protocol so that proc is protected, but doesn't take ->vl_servers_lock to
protect afs_start_vl_iteration() (which does actually take a shared lock).

Fix this by making afs_update_cell() take an exclusive lock when replacing
->vl_servers.

Fixes: 0a5143f2 ("afs: Implement VL server rotation")
Signed-off-by: David Howells <dhowells@redhat.com>