nfsd4: better estimate of getattr response size

We plan to use this estimate to decide whether or not to allow zero-copy
reads.  Currently we're assuming all getattr's are a page, which can be
both too small (ACLs e.g. may be arbitrarily long) and too large (after
an upcoming read patch this will unnecessarily prevent zero copy reads
in any read compound also containing a getattr).

Signed-off-by: J. Bruce Fields <bfields@redhat.com>