transposefs: Only autosave-xfs 1TiB filesystems

The change in coreos#2320
has been very problematic for OpenShift because our default node
configuration is *always* over the threshold, and that causes
significant latency on instance provisioning.

First, rework the reprovision threshold to operate in terms of
disk size, which is much easier to explain and debug than
allocation group count.  (Which to be clear, *is* the real problem,
but disk size is a good enough proxy for this)

Then, bump the reprovision threshold to 1TiB.  This is comfortably
about the default OpenShift node root disk sizes, and returns
us to the prior status quo.

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-ostree/ignition-ostree-transposefs.sh

@@ -138,14 +138,13 @@ should_autosave_rootfs() {
         echo 0
         return
     fi
-    local agcount
-    eval $(xfs_info "${root_part}" | grep -o 'agcount=[0-9]*')
-    # Semi-arbitrarily chosen: this is roughly ~64G currently (based on initial
-    # ag sizing at build time) which seems like a good rootfs size at which to
-    # discriminate between "throwaway/short-lived systems" and "long-running
-    # workload systems". It's not like XFS performance is way worse at 128.
-    if [ "$agcount" -lt 128 ]; then
-        echo "Filesystem agcount is $agcount; skipping" >&2
+    # Find rootfs size in KiB
+    local rootsize
+    local reprovisionsize
+    rootsize=$(($(stat -f -c '%b * %s ${root_part} 1024' /)))
+    reprovisionsize=$((1024**3))
+    if [ "$rootsize" -lt "$reprovisionsize" ]; then
+        echo "Filesystem size $rootsize KiB is less than threshold $reprovisionsize KiB; skipping" >&2
         echo 0
         return
     fi