Rename btrfs_root_is_snapshot to btrfs_root_is_snapper_snapshot.
This happens in preparation for the changes suggested in #2697
where we want to get rid of snapper specific btrfs code which
will be available in snapper natively soon. To make sure a btrfs
layout specific to snapper(and SUSE), the implicitly used attribute
named btrfs_root_is_snapshot now becomes explicit and its new
name will indicate that snapper sits behind it. Along with the
rename a XSLT stylesheet to automatically convert the old name
into the new name for schema v8.3 will be performed.
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