MDCache: "handle" request_forward on internal ops

MDCache: "handle" request_forward on internal ops

For now, just return -EXDEV ("Cross-device link") on internal ops that
require forwarding, as forwarding internal ops will require a great deal more
infrastructure.. But push the issue down to this level instead of worrying
about it in path_traverse, and consider the possibility that the MDRequest
might not have a client_request that it's wrapped around.

Signed-off-by: Greg Farnum <greg@inktank.com>