nfsd4: nfsv4 clients should cross mountpoints

Allow NFS v4 clients to seamlessly cross mount point without
have to set either the 'crossmnt' or the 'nohide' export
options.

Signed-Off-By: Steve Dickson <steved@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>