btrfs: rename rb_root member of extent_map_tree from map to root

Currently we name the rb_root member of struct extent_map_tree as 'map',
which is odd and confusing. Since it's a root node, rename it to 'root'.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>