NFSD: Reduce DRC bucket size

The common case is that a DRC lookup will not find the XID in the
bucket. Reduce the amount of pointer chasing during the lookup by
keeping fewer entries in each hash bucket.

Changing the bucket size constant forces the size of the DRC hash
table to increase, and the height of each bucket r-b tree to be
reduced.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>