The only remaining users are issuing SIOCGMIIPHY and SIOCGMIIREG,
neither of which deals with userland pointers.  Simply calling
->ndo_do_ioctl() is fine; no messing with set_fs() is needed.
It used to mess with SIOCETHTOOL, which would've needed set_fs(),
but that has been killed in "[NET] ethtool ops are the only way"
9 years ago...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>