Exclude zvol_misc_volmode for now

It keeps failing, on changes which aren't related at all.

So until someone runs down why, I'd like it to stop being the
sole reason for CI failures.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12733