OpenZFS 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.

Authored by: Matthew Ahrens <mahrens@delphix.com>
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>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Richard Lowe <richlowe@richlowe.net>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

Patch Notes:
* Removed ZFS_DEBUG_SPA from zfs-module-parameters.5

OpenZFS-issue: https://www.illumos.org/issues/9236
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/cfaba7f668
Closes #7467

module/zfs/spa_misc.c

@@ -248,7 +248,7 @@ int spa_mode_global;
  * by default in debug builds.
  */
 int zfs_flags = ~(ZFS_DEBUG_DPRINTF | ZFS_DEBUG_SET_ERROR |
-    ZFS_DEBUG_SPA | ZFS_DEBUG_INDIRECT_REMAP);
+    ZFS_DEBUG_INDIRECT_REMAP);
 #else
 int zfs_flags = 0;
 #endif
@@ -659,8 +659,6 @@ spa_add(const char *name, nvlist_t *config, const char *altroot)
 		    KM_SLEEP) == 0);
 	}
 
-	spa->spa_debug = ((zfs_flags & ZFS_DEBUG_SPA) != 0);
-
 	spa->spa_min_ashift = INT_MAX;
 	spa->spa_max_ashift = 0;
 
@@ -2122,12 +2120,6 @@ spa_scan_get_stats(spa_t *spa, pool_scan_stat_t *ps)
 	return (0);
 }
 
-boolean_t
-spa_debug_enabled(spa_t *spa)
-{
-	return (spa->spa_debug);
-}
-
 int
 spa_maxblocksize(spa_t *spa)
 {