afs_lookup(): switch to d_splice_alias()

->lookup() methods can (and should) use d_splice_alias() instead of
d_add().  Even if they are not going to be hit by open_by_handle(),
code does get copied around; besides, d_splice_alias() has better
calling conventions for use in ->lookup(), so the code gets simpler.

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