Mark Johnston a4834289d6 Handle NULL entries in gmirror disk ds_bios arrays.
Entries may be removed and freed if an I/O error occurs during mirror
synchronization, so we cannot assume that all entries of ds_bios are
valid.

Also ensure that a synchronization BIO's array index is preserved after
a successful write.

Reported and tested by:	pho
MFC after:	2 weeks
Sponsored by:	Dell EMC Isilon
2017-04-10 17:15:59 +00:00
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