nfsd4: fix rd_dircount enforcement

Commit 3b299709 "nfsd4: enforce rd_dircount" totally misunderstood
rd_dircount; it refers to total non-attribute bytes returned, not number
of directory entries returned.

Bring the code into agreement with RFC 3530 section 14.2.24.

Cc: stable@vger.kernel.org
Fixes: 3b299709

 "nfsd4: enforce rd_dircount"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>