When selecting a mirror child it's possible that map allocated by
vdev_mirror_map_allc() contains a NULL for the child vdev. In
this case the child should be skipped and the read issues to
another member of the mirror.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes#1744
33074f2254