Illumos 5213 - panic in metaslab_init due to space_map_open returning ENXIO

Reviewed by: Matthew Ahrens mahrens@delphix.com
Reviewed by: George Wilson george.wilson@delphix.com

References:
    https://www.illumos.org/issues/5213
    https://reviews.csiden.org/r/110

Ported by: Turbo Fredriksson <turbo@bayour.com>