gfs2: in signal_our_withdraw wait for unfreeze of _this_ fs only

gfs2: in signal_our_withdraw wait for unfreeze of _this_ fs only

Function signal_our_withdraw needs to work on file systems that have been
partially frozen. To do this, it called flush_workqueue(gfs2_freeze_wq).
This this wrong because it waits for *ALL* file systems to be unfrozen, not
just the one we're withdrawing from. It should only wait for the targetted
file system to be unfrozen. Otherwise it would wait until ALL file systems
are thawed before signaling the withdraw.

This patch changes signal_our_withdraw so it calls flush_work() for the target
file system's freeze work (only) to be completed.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>