nfsd: Revert "svcrpc: take advantage of tcp autotuning"

This reverts commit 47a14ef1 "svcrpc:
take advantage of tcp autotuning", which uncovered some further problems
in the server rpc code, causing significant performance regressions in
common cases.

We will likely reinstate this patch after releasing 2.6.30 and applying
some work on the underlying fixes to the problem (developed by Trond).

Reported-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Olga Kornievskaia <aglo@citi.umich.edu>
Cc: Jim Rees <rees@umich.edu>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>