netfilter: xt_qtaguid: fix error exit that would keep a spinlock.

qtudev_open() could return with a uid_tag_data_tree_lock held
when an kzalloc(..., GFP_ATOMIC) would fail.
Very unlikely to get triggered AND survive the mayhem of running out of mem.

Signed-off-by: JP Abgrall <jpa@google.com>
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>