GFS2: Extend umount wait coverage to full glock lifetime

Although all glocks are, by the time of the umount glock wait,
scheduled for demotion, some of them haven't made it far
enough through the process for the original set of waiting
code to wait for them.

This extends the ref count to the whole glock lifetime in order
to ensure that the waiting does catch all glocks. It does make
it a bit more invasive, but it seems the only sensible solution
at the moment.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>