Fix several performance related issues in the new NFS server's
DRC for NFS over TCP.
- Increase the size of the hash tables.
- Create a separate mutex for each hash list of the TCP hash table.
- Single thread the code that deletes stale cache entries.
- Add a tunable called vfs.nfsd.tcphighwater, which can be increased
  to allow the cache to grow larger, avoiding the overhead of frequent
  scans to delete stale cache entries.
  (The default value will result in frequent scans to delete stale cache
   entries, analagous to what the pre-patched code does.)
- Add a tunable called vfs.nfsd.cachetcp that can be used to disable
  DRC caching for NFS over TCP, since the old NFS server didn't DRC cache TCP.
It also adjusts the size of nfsrc_floodlevel dynamically, so that it is
always greater than vfs.nfsd.tcphighwater.

For UDP the algorithm remains the same as the pre-patched code, but the
tunable vfs.nfsd.udphighwater can be used to allow the cache to grow
larger and reduce the overhead caused by frequent scans for stale entries.
UDP also uses a larger hash table size than the pre-patched code.

Reported by:	wollman
Tested by:	wollman (earlier version of patch)
Submitted by:	ivoras (earlier patch)
Reviewed by:	jhb (earlier version of patch)
MFC after:	1 month