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Make g_attach() return ENXIO for orphaned providers; update various

Description

Make g_attach() return ENXIO for orphaned providers; update various
classes to add missing error checking.

Reviewed by: imp
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D26658

Details