Read backup GPT header from the last LBA only when primary GPT header and table aren't valid. If they are ok, use hdr_lba_alt value to read backup header. This will make gptboot happy when GPT used atop of some GEOM provider, e.g. GEOM_MIRROR.
Read backup GPT header from the last LBA only when primary GPT header and
table aren't valid. If they are ok, use hdr_lba_alt value to read backup
header. This will make gptboot happy when GPT used atop of some GEOM
provider, e.g. GEOM_MIRROR.

Reviewed by:	pjd
MFC after:	2 weeks

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