Clear btrfs file system super block mirror copies too (#705426)

With recent btrfs-progs, GParted failed to format a btrfs file system
over the top of an existing one.  Make btrfs failed with this error:

    # mkfs.btrfs /dev/sdb1
    ...
    /dev/sdb1 appears to contain an existing filesystem (btrfs).
    Use the -f option to force overwrite.

With this commit to btrfs-progs on 2013-02-19, mkfs.btrfs checks for
existing file system signatures, including all mirror copies of btrfs
super blocks, before writing to the partition.

    http://git.kernel.org/cgit/linux/kernel/git/mason/btrfs-progs.git/commit/?id=2a2d8e1962e8b6cda7b0a7584f6d2fb95d442cb6
    btrfs-progs: require mkfs -f force option to overwrite filesystem or partition table

Make GParted clear all the mirror copies of the btrfs file system super
blocks as erase_filesystem_signatures() is intended to prevent detection
of old signatures.  This also avoids having to determine if the -f
option to mkfs.btrfs is available before trying to use it.

Closes Bug #705426 - Formatting Existing BTRFS Partition as BTRFS Fails
                     Because mkfs.btrfs Is Not Run with "-f"