netfilter: x_tables: move sleeping allocation outside BH-disabled region

The jumpstack allocation needs to be moved out of the critical region.
Corrects this notice:

BUG: sleeping function called from invalid context at mm/slub.c:1705
[  428.295762] in_atomic(): 1, irqs_disabled(): 0, pid: 9111, name: iptables
[  428.295771] Pid: 9111, comm: iptables Not tainted 2.6.34-rc1 #2
[  428.295776] Call Trace:
[  428.295791]  [<c012138e>] __might_sleep+0xe5/0xed
[  428.295801]  [<c019e8ca>] __kmalloc+0x92/0xfc
[  428.295825]  [<f865b3bb>] ? xt_jumpstack_alloc+0x36/0xff [x_tables]

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>