During the rework of how xattr fields are handled, the comparison code
was not correctly updated. As a result, changes to xattrs would not be
detected in any form. This was detected in the umoci integration suite.
In addition, fix the dh.UsedKeywords logic so auto-detection works
correctly with prefix-based xattrs.
Fixes: ed464af779 ("*: xattr can Update()")
Signed-off-by: Aleksa Sarai <asarai@suse.de>
