10592 misc. metaslab and vdev related ZoL bug fixes
Portions contributed by: Jerry Jelinek <jerry.jelinek at joyent.com>
Reviewed by: Brian Behlendorf <behlendorf1 at llnl.gov>
Reviewed by: Giuseppe Di Natale <guss80 at gmail.com>
Reviewed by: George Melikov <mail at gmelikov.ru>
Reviewed by: Paul Dagnelie <pcd at delphix.com>
Reviewed by: Matt Ahrens <mahrens at delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov at delphix.com>
Reviewed by: Tony Hutter <hutter2 at llnl.gov>
Reviewed by: Kody Kantor <kody.kantor at joyent.com>
Approved by: Dan McDonald <danmcd at joyent.com>
UnifiedSplitRaw
