gfs2: Rename GLF_FREEING to GLF_UNLOCKED

Rename the GLF_FREEING flag to GLF_UNLOCKED, and the ->go_free glock
operation to ->go_unlocked.  This mechanism is used to wait for the
underlying DLM lock to be unlocked; being able to free the glock is a
consequence of the DLM lock being unlocked.

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