btrfs-progs: print-tree: Print offset as tree objectid for ROOT_ITEM

btrfs-progs: print-tree: Print offset as tree objectid for ROOT_ITEM

For cases like reloc trees and subvolume trees, their key offset is the
tree id.  The key will be printed as:

(TREE_RELOC ROOT_ITEM 18446744073709551607)

The negative number is long and even guys with real engineer brains
can't easily get the meaning.

This patch will change the output format to:

(TREE_RELOC ROOT_ITEM DATA_RELOC_TREE)

While for special offset value like 0 or (u64)-1, it's still shown as
is.

Signed-off-by: Qu Wenruo <wqu@suse.com>
[ reword comment ]
Signed-off-by: David Sterba <dsterba@suse.com>