afs: Make /afs/@cell and /afs/.@cell symlinks

Make /afs/@cell a symlink in the /afs dynamic root to match what other AFS
clients do rather than doing a substitution in the dentry name.  This has
the bonus of being tab-expandable also.

Further, provide a /afs/.@cell symlink to point to the dotted cell share.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20250107183454.608451-4-dhowells@redhat.com


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