Only bother walking a duplist if it has more than one element

Commit 5497f7e17cb2102e73a1d68c4685d9a8c58ace46 accidentally removed this
little bit of logic.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Fix leak of fiemap context in csum_whole_file()

We need to free this when done otherwise we were leaking quite a big buffer
(about 16k).

Signed-off-by: Mark Fasheh <mfasheh@suse.de>