udev: add /dev/loop/ symlinks

udev: add /dev/loop/ symlinks

This adds symlinks that allow accessing loopback block devices via stable
names that reference their backing block devices, make the unpredictable
naming of loopback devices less of an issue.

Example:

1. Create a loopback block device for a file $F

   losetup --find $F

2. Reference the backing block device via its inode:

   L="$(stat -c '/dev/loop/by-inode/%Hd:%Ld-%i' $F)"
   fdisk $L

In the above the loop device name (which might be /dev/loop47 or any
other name) is not used at all.