btrfs: relocation: open code mapping_tree_init

There's only one user of mapping_tree_init, we don't need a helper for
the simple initialization.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>