gfs2: Wake up when sd_glock_disposal becomes zero

commit da7d554f7c62d0c17c1ac3cc2586473c2d99f0bd upstream.

Commit fc0e38da ("GFS2: Fix glock deallocation race") fixed a
sd_glock_disposal accounting bug by adding a missing atomic_dec
statement, but it failed to wake up sd_glock_wait when that decrement
causes sd_glock_disposal to reach zero.  As a consequence,
gfs2_gl_hash_clear can now run into a 10-minute timeout instead of
being woken up.  Add the missing wakeup.

Fixes: fc0e38da ("GFS2: Fix glock deallocation race")
Cc: stable@vger.kernel.org # v2.6.39+
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>