gfs2: Add SDF_DEACTIVATING super block flag

Add a new SDF_DEACTIVATING super block flag that is set when the
filesystem has started to deactivate. This will be used in the next
patch to stop and drain the delete work during unmount.

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