isofs: avoid info leak on export

commit fe685aab upstream.

For type 1 the parent_offset member in struct isofs_fid gets copied
uninitialized to userland. Fix this by initializing it to 0.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>