libceph: make calc_target() set t->paused, not just clear it

In the Linux kernel, the following vulnerability has been resolved:

libceph: make calc_target() set t->paused, not just clear it

Currently calc_target() clears t->paused if the request shouldn't be

paused anymore, but doesn't ever set t->paused even though it's able to

determine when the request should be paused. Setting t->paused is left

to __submit_request() which is fine for regular requests but doesn't

work for linger requests -- since __submit_request() doesn't operate

on linger requests, there is nowhere for lreq->t.paused to be set.

One consequence of this is that watches don't get reestablished on

paused -> unpaused transitions in cases where requests have been paused

long enough for the (paused) unwatch request to time out and for the

subsequent (re)watch request to enter the paused state. On top of the

watch not getting reestablished, rbd_reregister_watch() gets stuck with

rbd_dev->watch_mutex held:

rbd_register_watch

__rbd_register_watch

ceph_osdc_watch

linger_reg_commit_wait

It's waiting for lreq->reg_commit_wait to be completed, but for that to

happen the respective request needs to end up on need_resend_linger list

and be kicked when requests are unpaused. There is no chance for that

if the request in question is never marked paused in the first place.

The fact that rbd_dev->watch_mutex remains taken out forever then

prevents the image from getting unmapped -- "rbd unmap" would inevitably

hang in D state on an attempt to grab the mutex.