afs: unmapping the wrong buffer

We switched from kmap_atomic() to kmap() so the kunmap() calls need to
be updated to match.

Fixes: d001648e

 ('rxrpc: Don't expose skbs to in-kernel users [ver #2]')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>