example/passthrough: Enable testing of readdirplus without fill offsets

example/passthrough: Enable testing of readdirplus without fill offsets

passthrough example supports the --plus command line argument to reply
to readdirplus with fill_dir_plus and unspecified (0) fill offsets.

As explained in this comment:
https://github.com/libfuse/libfuse/pull/896#issuecomment-1978917041
passthrough example needs a few more changes to be able to test commit
dd95d13a ("fix readdirplus when filler is called with zero offset (#896))

With the changes in this commit, readdirplus without fill offsets
can be tested to verify the readdirplus fix above with command line:

passthrough --plus -o auto_cache,modules=subdir,subdir=/src /mnt

Signed-off-by: Amir Goldstein <amir73il@gmail.com>