Fix a few kNFSd problems.

1/ export svc_reserve which was introduced for NFS/TCP support.
    Without this we cannot load nfsd.o as a module
2/ the hash chain of clients was being changed (to put the found
   entry at the top of the list) while we only had a read-lock.
   This could corrupt the list and cause big problems.
   For now, just disable this code.  Might add a lock later...
3/ lockd was calling exp_getclient without getting a readlock
   on the export table first.
4/ Add Config.help entry for CONFIG_NFSD_TCP