Drop references when skipping dmu_send due to EXDEV

Drop references when skipping dmu_send due to EXDEV

When an invalid incremental send is requested where the "to" ds is
before the "from" ds, make sure to drop the reference to the pool
and the dataset before returning the error.

Add an assert on FreeBSD to make sure we don't hold any locks after
returning from an ioctl.

Add some test coverage.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10919