Call zvol_create_minors() in spa_open_common() when initializing pool

Call zvol_create_minors() in spa_open_common() when initializing pool

There is an extremely odd bug that causes zvols to fail to appear on
some systems, but not others. Recently, I was able to consistently
reproduce this issue over a period of 1 month. The issue disappeared
after I applied this change from FreeBSD.

This is from FreeBSD's pool version 28 import, which occurred in
revision 219089.

Ported-by: Richard Yao <ryao@cs.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #441
Issue #599