futex: Return error code instead of assigning it without effect

The check on the rt_waiter and top_waiter->pi_state is assigning an error
return code to ret but this later gets re-assigned, hence the check is
ineffective.

Return -EINVAL rather than assigning it to ret which was the original
intent.

Fixes: dc7109aa ("futex: Validate waiter correctly in futex_proxy_trylock_atomic()")
Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: André Almeida <andrealmeid@collabora.com>
Link: https://lore.kernel.org/r/20210818131840.34262-1-colin.king@canonical.com