iplink: Use ll_index_to_name() instead of if_indextoname()

iplink: Use ll_index_to_name() instead of if_indextoname()

There are two reasons for switching to cached variant:

  1) ll_index_to_name() may return result from cache,
     eliminating expensive ioctl() to the kernel.

     Note that most of the code already switched from plain
     if_indextoname() to ll_index_to_name() to cached variant
     in print path because in most cases cache populated.

  2) It always return name in the form "if%d", even if
     entry is not in cache and ioctl() fails. This drops
     "link_index" from JSON output.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>