btrfs-progs: rebuild the crc tree with --init-csum-tree

btrfs-progs: rebuild the crc tree with --init-csum-tree

We have --init-csum-tree, which just empties the csum tree.  I'm not sure why we
would ever need this, but we definitely need to be able to rebuild the csum tree
in some cases.  This patch adds the ability to completely rebuild the crc tree
by reading all of the data and adding csum entries for them.  This patch doesn't
pay attention to NODATASUM inodes, it'll happily add csums for everything.
Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.cz>