sysfs: create optimal relative symlink targets

Instead of walking from the source down to the root of sysfs, and back
to the target, we stop at the first directory the source and the target
share.

This link:
  /devices/pci0000:00/0000:00:1d.7/usb1/1-0:1.0/ep_81

pointed to:
  ../../../../../devices/pci0000:00/0000:00:1d.0/usb2/2-0:1.0/usb_endpoint/usbdev2.1_ep81

now it just points to:
  usb_endpoint/usbdev1.1_ep81

Thanks to Denis Cheng for bringing this up, and sending the initial patch.

CC: Denis Cheng <crquan@gmail.com>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>