afs: Get an AFS3 ACL as an xattr

Implement an xattr on AFS files called "afs.acl" that retrieves a file's
ACL.  It returns the raw AFS3 ACL from the result of calling FS.FetchACL,
leaving any interpretation to userspace.

Note that whilst YFS servers will respond to FS.FetchACL, this will render
a more-advanced YFS ACL down.  Use "afs.yfs.acl" instead for that.

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