nfsd: replace boolean nfsd_up flag by users counter

Since we have generic NFSd resurces, we have to introduce some way how to
allocate and destroy those resources on first per-net NFSd start and on
last per-net NFSd stop respectively.
This patch replaces global boolean nfsd_up flag (which is unused now) by users
counter and use it to determine either we need to allocate generic resources
or destroy them.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>