Use log_must_busy in destroy_pool

The log function log_must_busy was added in commit e623aea2 for
this purpose. Update destroy_pool to use it.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6217