i40evf_add_vlan() is never called in atomic context.

i40evf_add_vlan() is only called by i40evf_vlan_rx_add_vid(),
which is only set as ".ndo_vlan_rx_add_vid" in struct net_device_ops.
".ndo_vlan_rx_add_vid" is not called in atomic context.

Despite never getting called from atomic context,
i40evf_add_vlan() calls kzalloc() with GFP_ATOMIC,
which does not sleep for allocation.
GFP_ATOMIC is not necessary and can be replaced with GFP_KERNEL,
which can sleep and improve the possibility of sucessful allocation.

This is found by a static analysis tool named DCNS written by myself.
And I also manually check it.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>