net-sysfs: require net admin in the init ns for setting tx_maxrate

An upcoming change will allow container root to open some /sys/class/net
files for writing. The tx_maxrate attribute can result in changes
to actual hardware devices so err on the side of caution by requiring
CAP_NET_ADMIN in the init namespace in the corresponding attribute store
operation.

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>