btrfs-progs: Allow btrfs_read_dev_super() to read all 3 super for super_recover.

btrfs-progs: Allow btrfs_read_dev_super() to read all 3 super for super_recover.

Btrfs-progs superblock checksum check is somewhat too restricted for
super-recover, since current btrfs-progs will only read the 1st
superblock and if you need super-recover the 1st superblock is
possibly already damaged.

The fix is introducing super_recover parameter for
btrfs_read_dev_super() and callers to allow scan backup superblocks if
needed.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>