Enable tuning of ZVOL open timeout value

The default timeout for ZVOL opens may not be sufficient for all cases,
so we should enable the value to be more easily tuned to account for
systems where the default value is insufficient.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Prakash Surya <prakash.surya@delphix.com>
Closes #15023