net: core: decouple ifalias get/set from rtnl lock

Device alias can be set by either rtnetlink (rtnl is held) or sysfs.

rtnetlink hold the rtnl mutex, sysfs acquires it for this purpose.
Add an extra mutex for it and use rcu to protect concurrent accesses.

This allows the sysfs path to not take rtnl and would later allow
to not hold it when dumping ifalias.

Based on suggestion from Eric Dumazet.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>