per-tagset SRCU struct and quiesce

[Sagi Grimberg]

> Hi all,
> 
> this series moves the SRCU struct used for quiescing to the tag_set
> as the SRCU critical sections for dispatch take about the same time
> on all queues anyway, and then adopts the series from Chao that provides
> tagset-wide quiesce to use that infrastructure.

Looks nice, should be easy enough to modify scsi_host_block() to
use this as well.