Fix a build warning about leaking CONFIG_NFSD to userspace.

Fix a build warning about leaking CONFIG_NFSD to userspace.

The nfsd_stats data structure does not need to be available to
userspace; no kernel interface uses it.  So move it inside #ifdef
__KERNEL__ and the warning goes away.

Signed-off-by: Greg Banks <gnb@sgi.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>