net: rmnet: use GFP_KERNEL instead of GFP_ATOMIC

In the current code, rmnet_register_real_device() and rmnet_newlink()
are using GFP_ATOMIC.
But, these functions are allowed to sleep.
So, GFP_KERNEL can be used.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>