Martin Matuska 833aeb586b Return EIO if vdev->v_phys_read is NULL.
This fixes booting from a ZFS mirror with a unavailable primary device.

PR:		kern/148655
Reviewed by:	avg
Approved by:	delphij (mentor)
MFC after:	3 days
2010-08-09 06:36:11 +00:00
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