nfsd: further centralize protocol version checks.

With this patch the only places that test ->rq_vers against a specific
version are nfsd_v4client() and nfsd_set_fh_dentry().
The latter sets some flags in the svc_fh, which now includes:
  fh_64bit_cookies
  fh_use_wgather

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>