virtio-blk: don't keep queue frozen during system suspend

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

virtio-blk: don't keep queue frozen during system suspend

Commit 4ce6e2db00de ("virtio-blk: Ensure no requests in virtqueues before

deleting vqs.") replaces queue quiesce with queue freeze in virtio-blk's

PM callbacks. And the motivation is to drain inflight IOs before suspending.

block layer's queue freeze looks very handy, but it is also easy to cause

deadlock, such as, any attempt to call into bio_queue_enter() may run into

deadlock if the queue is frozen in current context. There are all kinds

of ->suspend() called in suspend context, so keeping queue frozen in the

whole suspend context isn't one good idea. And Marek reported lockdep

warning[1] caused by virtio-blk's freeze queue in virtblk_freeze().

[1] https://lore.kernel.org/linux-block/[email protected]/

Given the motivation is to drain in-flight IOs, it can be done by calling

freeze & unfreeze, meantime restore to previous behavior by keeping queue

quiesced during suspend.