losetup: add --associated option

losetup: add --associated option

    losetup -j | --associated <file> [-o <num>]

This new option lists all loop devices associated with given file
(and optionally file offset).

Examples:

 # losetup -a
 /dev/loop0: [0807]:21921808 (/foo/vfat-mbr.img)
 /dev/loop1: [0807]:21921808 (/foo/vfat-mbr.img), offset 100
 /dev/loop2: [0807]:21921802 (/bar/ext3.img)

 # losetup -j /foo/vfat-mbr.img
 /dev/loop0: [0807]:21921808 (/foo/vfat-mbr.img)
 /dev/loop1: [0807]:21921808 (/foo/vfat-mbr.img), offset 100

 # losetup -j /foo/vfat-mbr.img -o 100
 /dev/loop1: [0807]:21921808 (/foo/vfat-mbr.img), offset 100

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