afs: Add rootcell checks

afs: Add rootcell checks

Add some checks for the validity of the cell name.  It's may get put into a
symlink, so preclude it containing any slashes or "..".  Also disallow
starting/ending with a dot.  This makes /afs/@cell/ as a symlink less of a
security risk.

Also disallow multiple setting of /proc/net/afs/rootcell for any given
network namespace.  Once set, the value may not be changed.  This makes it
easier to only create /afs/@cell and /afs/.@cell if there's a rootcell.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20250107183454.608451-3-dhowells@redhat.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>