nfsd41: hange from page to memory based drc limits

NFSD_SLOT_CACHE_SIZE is the size of all encoded operation responses
(excluding the sequence operation) that we want to cache.

For now, keep NFSD_SLOT_CACHE_SIZE at PAGE_SIZE. It will be reduced
when the DRC is changed from page based to memory based.

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