net-sysfs: convert netstat_show() to RCU

dev_get_stats() can be called from RCU, there is no need
to acquire dev_base_lock.

Change dev_isalive() comment to reflect we no longer use
dev_base_lock from net/core/net-sysfs.c

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>