null_blk: improve zone locking

With memory backing disabled, using a single spinlock for protecting
zone information and zone resource management prevents the parallel
execution on multiple queue of IO requests to different zones.
Furthermore, regardless of the use of memory backing, if a null_blk
device is created without limits on the number of open and active zones,
accounting for zone resource management is not necessary.

>From these observations, zone locking is changed as follows to improve
performance:
1) the zone_lock spinlock is renamed zone_res_lock and used only if
   zone resource management is necessary, that is, if either
   zone_max_open or zone_max_active are not 0. This is indicated using
   the new boolean need_zone_res_mgmt in the nullb_device structure.
   null_zone_write() is modified to reduce the amount of code executed
   with the zone_res_lock spinlock held.
2) With memory backing disabled, per zone locking is changed to a
   spinlock per zone.
3) Introduce the structure nullb_zone to replace the use of
   struct blk_zone for zone information. This new structure includes a
   union of a spinlock and a mutex for zone locking. The spinlock is
   used when memory backing is disabled and the mutex is used with
   memory backing.

With these changes, fio performance with zonemode=zbd for 4K random
read and random write on a dual socket (24 cores per socket) machine
using the none schedulder is as follows:

before patch:
	write (psync x 96 jobs) = 465 KIOPS
	read (libaio@qd=8 x 96 jobs) = 1361 KIOPS
after patch:
	write (psync x 96 jobs) = 456 KIOPS
	read (libaio@qd=8 x 96 jobs) = 4096 KIOPS

Write performance remains mostly unchanged but read performance is three
times higher. Performance when using the mq-deadline scheduler is not
changed by this patch as mq-deadline becomes the bottleneck for a
multi-queue device.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>