RFE: mount newly formatted volumes with barrier=0

[Avi Kivity <avi redhat com>]

If Anaconda has just formatted a volume, we know that there is no data 
on that volume that can be lost.  We can therefore mount it, for the 
duration of the installation, with barriers disabled (and make other 
tradeoffs in the performance/integrity equation, like using 
data=writeback for ext4).

The installed image should enable barriers as usual.

The result would be faster installation, and thus a better user experience.

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