nfsd: recovery - make rec_file per net

Opening and closing of this file is done in client tracking init and exit
operations.
Client tracking is done in network namespace context already. So let's make
this file opened and closed per network context - this will simlify it's
management.

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