Avoid Linux hung task message in ZTHR

Use an interruptible to avoid Linux hung task message in
ZTHR and to prevent inflating the load average.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #7440
Closes #7441