3954 metaslabs continue to load even after hitting zfs_mg_alloc_failure limit
4080 zpool clear fails to clear pool
4081 need zfs_mg_noalloc_threshold
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>

*** 169,178 ****
--- 169,179 ----
  
          /*
           * The zio write taskqs have 1 thread per cpu, allow 1/2 of the taskqs
           * to fail 3 times per txg or 8 failures, whichever is greater.
           */
+         if (zfs_mg_alloc_failures == 0)
                  zfs_mg_alloc_failures = MAX((3 * max_ncpus / 2), 8);
  
          zio_inject_init();
  }
  
*** 2363,2373 ****
          }
  
          if (error) {
                  error = metaslab_alloc(spa, spa_normal_class(spa), size,
                      new_bp, 1, txg, old_bp,
!                     METASLAB_HINTBP_AVOID | METASLAB_GANG_AVOID);
          }
  
          if (error == 0) {
                  BP_SET_LSIZE(new_bp, size);
                  BP_SET_PSIZE(new_bp, size);
--- 2364,2374 ----
          }
  
          if (error) {
                  error = metaslab_alloc(spa, spa_normal_class(spa), size,
                      new_bp, 1, txg, old_bp,
!                     METASLAB_HINTBP_AVOID);
          }
  
          if (error == 0) {
                  BP_SET_LSIZE(new_bp, size);
                  BP_SET_PSIZE(new_bp, size);