nfp: flower: vxlan - ensure no sleep in atomic context

Functions called by the netevent notifier must be in atomic context.
Change the mutex to spinlock and ensure mem allocations are done with the
atomic flag.
Also, remove unnecessary locking after notifiers are unregistered.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>