nfsd41: sanity check client drc maxreqs

Ensure the client requested maximum requests are between 1 and
NFSD_MAX_SLOTS_PER_SESSION

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>