afs: Make /afs/.<cell> as well as /afs/<cell> mountpoints

When a cell is instantiated, automatically create an /afs/.<cell>
mountpoint to match the /afs/<cell> mountpoint to match other AFS clients.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20250107183454.608451-2-dhowells@redhat.com


cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>