nfsd41: define NFSD_DRC_SIZE_SHIFT in set_max_drc

Fixes the following compiler error:
fs/nfsd/nfssvc.c: In function 'set_max_drc':
fs/nfsd/nfssvc.c:240: error: 'NFSD_DRC_SIZE_SHIFT' undeclared

CONFIG_NFSD_V4 is not set

Reported-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>