Datasets can be busy when calling zfs destroy. Introduce
a helper function to destroy datasets and use it to destroy
datasets in zfs_allow_004_pos, zfs_promote_008_pos, and
zfs_destroy_002_pos.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes#7224Closes#7246Closes#7249Closes#7267