Make the nfsd read-ahead params cache more SMP-friendly by changing the single
global list and lock into a fixed 16-bucket hashtable with per-bucket locks.
This reduces spinlock contention in nfsd_read() on read-heavy workloads on
multiprocessor servers.

Testing was on a 4 CPU 4 NIC Altix using 4 IRIX clients each doing 1K
streaming reads at full line rate.  The server had 128 nfsd threads, which
sizes the RA cache at 256 entries, of which only a handful were used.  Flat
profiling shows nfsd_read(), including the inlined nfsd_get_raparms(), taking
10.4% of each CPU.  This patch drops the contribution from nfsd() to 1.71% for
each CPU.
Signed-off-by: Greg Banks <gnb@melbourne.sgi.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>