NFSd: introduce nfsd_destroy() helper

commit 19f7e2ca44dfc3c1b3f499fc46801f98d403500f upstream.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>