Revert "kill dev_ifsioc()"

This reverts commit bf440573 ("kill dev_ifsioc()").

This wasn't really unused as implied by the original commit,
it still handles the copy to/from user differently, and the
commit thus caused issues such as
  https://bugzilla.kernel.org/show_bug.cgi?id=199469
and
  https://bugzilla.kernel.org/show_bug.cgi?id=202273

However, deviating from a strict revert, rename dev_ifsioc()
to compat_ifreq_ioctl() to be clearer as to its purpose and
add a comment.

Cc: stable@vger.kernel.org
Fixes: bf440573

 ("kill dev_ifsioc()")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>