LockD: manage garbage collection timeout per networks namespace

This patch moves next_gc to per-net data.

Note: passed network can be NULL (when Lockd kthread is exiting of Lockd
module is removing).

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

diff --cc fs/lockd/host.c
Simple merge

diff --cc fs/lockd/netns.h
Simple merge