btrfs-progs: check: add support to clear v1 free space cache

btrfs-progs: check: add support to clear v1 free space cache

Kernel clear_cache mount option will only rebuild free space cache if
the used space of that chunk has changed.

So it won't ensure any corrupted free space cache get cleared.

So add a new option "--clear-space-cache v1|v2" to btrfsck, to
completely wipe out free space cache.
So kernel won't complain again.

Reported-by: Ivan P <chrnosphered@gmail.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
[ adjusted error messages, doc wording changes ]
Signed-off-by: David Sterba <dsterba@suse.com>