udf: avoid info leak on export

commit 0143fc5e9f6f5aad4764801015bc8d4b4a278200 upstream.

For type 0x51 the udf.parent_partref member in struct 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>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>