Add xattrs to allow the user to get/set metadata in lieu of having pioctl()
available.  The following xattrs are now available:

 (*) afs.cell

     The name of the cell in which the vnode's volume resides.

 (*) afs.fid

     The volume ID, vnode ID and vnode uniquifier of the file as three hex
     numbers separated by colons.

 (*) afs.volume

     The name of the volume in which the vnode resides.

For example:

	# getfattr -d -m ".*" /mnt/scratch
	getfattr: Removing leading '/' from absolute path names
	# file: mnt/scratch
	afs.cell="mycell.myorg.org"
	afs.fid="10000b:1:1"
	afs.volume="scratch"
Signed-off-by: David Howells <dhowells@redhat.com>