afs: Implement @cell substitution handling

Implement @cell substitution handling such that if @cell is seen as a name
in a dynamic root mount, then the name of the root cell for that network
namespace will be substituted for @cell during lookup.

The substitution of @cell for the current net namespace is set by writing
the cell name to /proc/fs/afs/rootcell.  The value can be obtained by
reading the file.

For example:

	# mount -t afs none /kafs -o dyn
	# echo grand.central.org >/proc/fs/afs/rootcell
	# ls /kafs/@cell
	archive/  cvs/  doc/  local/  project/  service/  software/  user/  www/
	# cat /proc/fs/afs/rootcell
	grand.central.org

Signed-off-by: David Howells <dhowells@redhat.com>