gfs2: Do not cancel internal demote requests

gfs2: Do not cancel internal demote requests

Trying to change the state of a glock may result in a "conversion
deadlock" error, indicating that the requested state transition would
cause a deadlock.  In this case, we unlock and retry the state change.
It makes no sense to try canceling those unlock requests, but the
current code is not aware of that.

In addition, if a locking request is canceled shortly after it is made,
the cancelation request can currently overtake the locking request.
This may result in deadlocks.

Fix both of these bugs by repurposing the GLF_PENDING_REPLY flag into a
GLF_MAY_CANCEL flag which is set only when the current locking request
can be canceled.  When trying to cancel a locking request in
gfs2_glock_dq(), wait for this flag to be set.

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