MFC r336956: MFV r336955: 9236 nuke spa_dbgmsg

MFC r336956: MFV r336955: 9236 nuke spa_dbgmsg

We should use zfs_dbgmsg instead of spa_dbgmsg.  Or at least,
metaslab_condense() should call zfs_dbgmsg because it's important and rare
enough to always log. It's possible that the message in zio_dva_allocate()
would be too high-frequency for zfs_dbgmsg.

illumos/illumos-gate@21f7c81cc1156e9202ce3412d3ecaa697c3b2222

Reviewed by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Matthew Ahrens <mahrens@delphix.com>