btrfs: remove duplicated uuid tree existence check in btrfs_uuid_tree_add()

btrfs: remove duplicated uuid tree existence check in btrfs_uuid_tree_add()

There's no point in checking if the uuid root exists in
btrfs_uuid_tree_add(), since we already do it in btrfs_uuid_tree_lookup().
We can just remove the check from btrfs_uuid_tree_add() and make
btrfs_uuid_tree_lookup() return -EINVAL instead of -ENOENT in case the
uuid tree does not exists.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>