nfsd: clarify readdir/mountpoint-crossing code

The code here is difficult to understand; attempt to clarify somewhat by
pulling out one of the more mystifying conditionals into a separate
function.

While we're here, also add lease_time to the list of attributes that we
don't really need to cross a mountpoint to fetch.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Peter Staubach <staubach@redhat.com>