btrfs: qgroup: remove duplicated check in adding qgroup relations

Removes duplicated check when adding qgroup relations.
btrfs_add_qgroup_relations function adds relations by calling
add_relation_rb(). add_relation_rb() checks that member/parentid exists
in current qgroup_tree. But it already checked before calling the
function. It seems that we don't need to double check.

Add new function __add_relation_rb() that adds relations with
qgroup structures and makes old function use the new one. And it makes
btrfs_add_qgroup_relation() function work without double checks by
calling the new function.

Signed-off-by: Sidong Yang <realwakka@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
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Signed-off-by: David Sterba <dsterba@suse.com>