MFC r270973

r270973
Fix for WITNESS warning while doing xpt_rescan.
This happen when converting any JBOD to RAID or creating
any new RAID from Unconfigured Drives.

Without this fix, user may see call trace if  WITNESS is enabled.
System may panic when reconfiguring the RAID.

Reviewed by: ambrisko
Approved by: re (gleb)

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Merge remote-tracking branch 'freebsd/stable/10' into hardened/10/aslr