[NET]: split dev_ifsioc() according to locking

This always bugged me: dev_ioctl() called dev_ifsioc() either inside
read_lock(dev_base_lock) or rtnl_lock(), depending on the ioctl being
executed.

This change moves the ioctls executed inside dev_base_lock to a new
function, dev_ifsioc_locked().  Now the locking context is completely
clear to the reader.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>