[2.1] spa_min_alloc should be GCD, not min

[ixhamza]

Since spa_min_alloc may not be a power of 2, unlike ashifts, in the case of DRAID, we should not select the minimal value among several vdevs. Rounding to a multiple of it is unlikely to work for other vdevs. Instead, using the greatest common divisor produces smaller yet more reasonable results.

Reviewed-by: Brian Behlendorf behlendorf1@llnl.gov
Reviewed-by: Alexander Motin mav@FreeBSD.org
Signed-off-by: Ameer Hamza ahamza@ixsystems.com
Closes openzfs#15067

Motivation and Context

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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• My code follows the OpenZFS code style requirements.
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