NFSv4: Ensure non-root user can trigger a referral automount

Currently only root can trigger a referral automount because only root
can access rpc_pipefs directories. Enabling read access to non-root
should be harmless (they can still not access the pipes themselves)
and will suffice to fix this problem.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>