NFSD: Fix nfsdcld warning

Since CONFIG_NFSD_LEGACY_CLIENT_TRACKING is a new config option, its
initial default setting should have been Y (if we are to follow the
common practice of "default Y, wait, default N, wait, remove code").

Paul also suggested adding a clearer remedy action to the warning
message.

Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Message-Id: <d2ab4ee7-ba0f-44ac-b921-90c8fa5a04d2@molgen.mpg.de>
Fixes: 74fd4873

 ("nfsd: new Kconfig option for legacy client tracking")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>