makefs: zfs uberblock location is calculated wrong

[tsoome]

Summary

The shift used to calculate uberblock location depends both
on minimum size (UBERBLOCK_SHIFT) and MAX_UBERBLOCK_SHIFT.
Since makefs defaults to use ashift 12, it incidentally
does get the correct size, but ashift 9 does not work with
current code.

[markj]

It'd be nice to have a helper macro for this, basically the same as VDEV_UBERBLOCK_SHIFT but just taking an integer ashift as a parameter. There is already one such macro in OpenZFS, ASHIFT_UBERBLOCK_SHIFT in zhack.c.

[tsoome]

use ASHIFT_UBERBLOCK_SIZE().