MFC r254077: MFV r254071: Fix a regression introduced by fix for Illumos bug #3834
MFC r254112: MFV r254079: multiple ZFS issues
MFC r254112: MFV r254079: multiple ZFS issues
MFC r255750: MFV r254750: Add support of Illumos dumps on zvol over RAID-Z.

Note that this only adds the features.  FreeBSD would
still need more work to support dumping on zvols.

MFC slacker:	delphij
MFC r258717: MFV r258371,r258372: 4101 metaslab_debug should allow for
fine-grained control