initscripts: run umountnfs as a KILL script

`rc` runs all the KILL scripts in a runlevel before the START scripts.
The umountnfs script is currently configured as a START script, and
runs after the networking KILL script. During shutdown, this causes a
~3 minute timeout after networking is shutdown when the system tries
to connect to and unmount any mounted network shares.
Fix this by changing the script configuration to "stop" so that it can
run before networking is stopped and unmount any network shares
safely.

Signed-off-by: Shruthi Ravichandran <shruthi.ravichandran@ni.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>