afs: Fix unchecked-length string display in debug statement

Fix afs_extract_vlserver_list() to limit the length of the displayed
string in a debug statement().

Fixes: 0a5143f2f89c ("afs: Implement VL server rotation")
Closes: https://sashiko.dev/#/patchset/20260618074903.2374756-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org