ipf_nat_getent() allocates a variable-sized nat_save_t buffer with
KMALLOCS() (which does not zero memory) and then copies only a subset
of fields into it before returning the object to userland using
ipf_outobjsz(). Because the structure is not fully initialized on all
paths, uninitialized kernel heap bytes can be copied back to user space,
resulting in an information leak.

We fix this by zeroing out the data structure immediately after
allocation.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
MFC after: 1 day