nfsd: allow nfsv3 readdir request to be larger.

nfsd currently reports the NFSv3 dtpref FSINFO parameter
to be PAGE_SIZE, so NFS clients will typically ask for one
page of directory entries at a time.  This is needlessly restrictive
as nfsd can handle larger replies easily.

Also, a READDIR request (but not a READDIRPLUS request) has the count
size clipped to PAGE_SIE, again unnecessary.

This patch lifts these limits so that larger readdir requests can be
used.

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