gfs2: Fix additional unlikely request cancelation race

In gfs2_glock_dq(), we must drop the glock spin lock before calling
->lm_cancel, but this means that in the meantime, the operation we are
trying to cancel could complete.  If the operation completes
unsuccessfully, another holder can end up at the head of the queue and
another ->lm_lock operation can get started.  In this case, we would end
up canceling that second operation by accident.

To prevent that, introduce a new GLF_CANCELING flag.  Set that flag in
gfs2_glock_dq() when trying to cancel an operation.  When seeing that
flag, finish_xmote() will then keep the GLF_LOCK flag set to prevent
other glock operations from taking place.  gfs2_glock_dq() then
completes the cancelation attempt by clearing GLF_LOCK and
GLF_CANCELING.

In addition, add a missing GLF_DEMOTE_IN_PROGRESS check in
gfs2_glock_dq() to make sure that we won't accidentally cancel a demote
request.

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