partname: Introduce fstools_partname_fallback_scan option

partname: Introduce fstools_partname_fallback_scan option

Some device may contains a GPT partition named rootfs_data that may
not be suitable.
To save from regression with old implementation that doesn't use
fstools_ignore_partname to explicitly say that that parname scan
should be ignored, make explicit that scanning each partition should
be done by providing fstools_partname_fallback_scan=1 and skip partname
scan in every other case.

Fixes: e9b59f063bb3 ("partname: Ignore root=PARTUUID...")
Tested-by: Dirk Buchwalder <buchwalder@posteo.de>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>