net/nlbwmon: run with lower priority

net/nlbwmon: run with lower priority

Even on a powerful platform a nlbwmon process'
activities are sometimes affecting throoughput and
latency. This is a backgroud process, that should not
be running with default priority.
Even if it is a little deplayed, that is not a worry in
this case. The routing should be the main priority,
bandwidth stats collection can wait a bit.

Tested on Netgear R7800
Signed-off-by: Marc Benoit <marcb62185@gmail.com>
(cherry picked from commit 9b3ecbd64a6328e13981165660d8930e8bb2446a)