Make dsl_scan print the pool name in dbgmsg

If you've got multiple scrubs/resilvers going, it's rather helpful
to know which pool each scan line refers to.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes: #12674