btrfs-progs: dump-tree: add option to print children nodes of a given block

btrfs-progs: dump-tree: add option to print children nodes of a given block

When debuging with "btrfs inspect dump-tree", it's not that handy if we
want to iterate all child tree blocks starting from a specified block.

-b can only print a single block, while without -b "btrfs inspect dump-tree"
will need extra tree roots fulfilled to continue, which is not possible
for a damaged filesystem.

Add a new option --follow to iterate a sub-tree starting from block
specified by --block.

Signed-off-by: Qu Wenruo <wqu@suse.com>
[ remove the short option for now ]
Signed-off-by: David Sterba <dsterba@suse.com>