netfilter: qtaguid: Don't BUG_ON if create_if_tag_stat fails

If create_if_tag_stat fails to allocate memory (GFP_ATOMIC) the
following will happen:

qtaguid: iface_stat: tag stat alloc failed
...
kernel BUG at xt_qtaguid.c:1482!

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>