net: sfc: Replace in_interrupt() usage

efx_ef10_try_update_nic_stats_vf() used in_interrupt() to figure out
whether it is safe to sleep (for MCDI) or not.

The only caller from which it was not is efx_net_stats(), which can be
invoked under dev_base_lock from net-sysfs::netstat_show().

So add a new update_stats_atomic() method to struct efx_nic_type, and call
it from efx_net_stats(), removing the need for
efx_ef10_try_update_nic_stats_vf() to behave differently for this case
(which it wasn't doing correctly anyway).

For all nic_types other than EF10 VF, this method is NULL so the the
regular update_stats() methods are invoked , which are happy with being
called from atomic contexts.

Fixes: f00bf230 ("sfc: don't update stats on VF when called in atomic context")
Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Martin Habets <mhabets@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>