net/smc: use GFP_ATOMIC allocation in smc_pnet_add_eth()

My last patch moved the netdev_tracker_alloc() call to a section
protected by a write_lock().

I should have replaced GFP_KERNEL with GFP_ATOMIC to avoid the infamous:

BUG: sleeping function called from invalid context at include/linux/sched/mm.h:256

Fixes: 28f92221

 ("net/smc: fix ref_tracker issue in smc_pnet_add()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>