net: fix rtnl notification in atomic context

commit 991fb3f7 "dev: always advertise rx_flags changes via netlink"
introduced rtnl notification from __dev_set_promiscuity(),
which can be called in atomic context.

Steps to reproduce:
ip tuntap add dev tap1 mode tap
ifconfig tap1 up
tcpdump -nei tap1 &
ip tuntap del dev tap1 mode tap

[  271.627994] device tap1 left promiscuous mode
[  271.639897] BUG: sleeping function called from invalid context at mm/slub.c:940
[  271.664491] in_atomic(): 1, irqs_disabled(): 0, pid: 3394, name: ip
[  271.677525] INFO: lockdep is turned off.
[  271.690503] CPU: 0 PID: 3394 Comm: ip Tainted: G        W    3.12.0-rc3+ #73
[  271.703996] Hardware name: System manufacturer System Product Name/P8Z77 WS, BIOS 3007 07/26/2012
[  271.731254]  ffffffff81a58506 ffff8807f0d57a58 ffffffff817544e5 ffff88082fa0f428
[  271.760261]  ffff8808071f5f40 ffff8807f0d57a88 ffffffff8108bad1 ffffffff81110ff8
[  271.790683]  0000000000000010 00000000000000d0 00000000000000d0 ffff8807f0d57af8
[  ...