mountd: better hiding of v4root exports from mountd clients

We've hidden v4root exports from get_exportlist (hence from the
showmount command), but not from other mountd operations--allowing
clients to attempt to mount exports when they should be getting an
immediate error.

Symptoms observed on a linux client were that a mount that previously
would have returned an error immediately now hung.  This restores the
previous behavior.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>