DDT: Reduce global DDT lock scope during writes

Before this change DDT lock was taken 4 times per written block,
and as effectively a pool-wide lock it can be highly congested.
This change introduces a new per-entry dde_io_lock, protecting some
fields during I/O ready and done stages, so that we don't need the
global lock there.

According to my write tests on 64-thread system with 4KB blocks this
significantly reduce the global lock contention, reducing CPU usage
from 100% to expected ~80%, and increasing write throughput by 10%.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17960