losetup: use /sys/dev/block/.../loop/backing_file

losetup: use /sys/dev/block/.../loop/backing_file

The basic loopdev attributes are available in sysfs since kernel
2.6.37. This patch uses the backing_file attribute from sysfs for very
long filenames (the LOOP_GET_STATUS ioctl uses only 64 bytes for the
filename).

old version:
 # losetup -a
 /dev/loop0: [0804]:12865322 (/home/images/filesystems/this_is_really_really_long_directory_*)

new version:
 # losetup -a
 /dev/loop0: [0804]:12865322 (/home/images/filesystems/this_is_really_really_long_directory_name/ext2.img)

Signed-off-by: Karel Zak <kzak@redhat.com>