Btrfs-progs: add super-recover to recover bad supers

Until now if one of device's first superblock is corrupt,btrfs will
fail to mount. Luckily, btrfs have at least two superblocks for
every disk.

In theory, if silent corrupting happens when we are writting superblocks
into disk, we must hold at least one good superblock.

One side effect is that user must gurantee that the disk must be
a btrfs disk. Otherwise, this tool may destroy other fs.(This is also
reason why btrfs only use first superblock in every disk to mount)

This little program will try to correct bad superblocks from
good superblocks with max generation.

There will be five kinds of return values:

0: all supers are valid, no need to recover
1: usage or syntax error
2: recover all bad superblocks successfully
3: fail to recover bad superblocks
4: abort to recover bad superblocks

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>