AFS: Use i_generation not i_version for the vnode uniquifier

Store the AFS vnode uniquifier in the i_generation field, not the i_version
field of the inode struct.  i_version can then be given the AFS data version
number.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>