libblkid: don't mark cache as "probed" if /sys not available

libblkid: don't mark cache as "probed" if /sys not available

For "mount --all" we need to read the cache more than once in a short
time. The library checks the delay between probes, and if the delay is
too short, it does not read devices. This is a problem on boot when there
are no /sys, and the cache is empty. In this case, we need to check
for /sys until it's available constantly.

https://github.com/util-linux/util-linux/issues/1492
Signed-off-by: Karel Zak <kzak@redhat.com>