CVE-2026-72308: mlxsw: fix refcount leak in mlxsw_sp_port_lag_join()

In the Linux kernel, the following vulnerability has been resolved:

mlxsw: fix refcount leak in mlxswspportlagjoin()

When mlxswspportlagindexget() fails, mlxswspportlagjoin() returns an error without releasing the lag reference obtained by the earlier mlxswsplagget(). All other error paths in the function jump to the cleanup label that ends with mlxswsplagput(), so this is a single missed release.

Fix the leak by replacing the bare 'return err' with a goto to the existing error cleanup label, which will drop the reference safely.