rpc_pipe: set dentry operations at d_alloc time

Currently the way these get set is a little convoluted. If the dentry is
allocated via lookup from userland, then it gets set by simple_lookup.
If it gets allocated when the kernel is populating the directory, then
it gets set via __rpc_lookup_create_exclusive, which has to check
whether they might already be set. Between both of these, this ensures
that all dentries have their d_op pointer set.

Instead of doing that, just have them set at d_alloc time by pointing
sb->s_d_op at them. With that change, we no longer want the lookup op
to set them, so we must move to using our own lookup routine.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>