selftests/futex: getopt() requires int as return value.

Mark reported that futex_priv_hash fails on ARM64.
It turns out that the command line parsing does not terminate properly
and ends in the default case assuming an invalid option was passed.

Use an int as the return type for getopt().

Closes: https://lore.kernel.org/all/31869a69-063f-44a3-a079-ba71b2506cce@sirena.org.uk/


Fixes: 31633694 ("selftests/futex: Add futex_numa_mpol")
Fixes: cda95fae ("selftests/futex: Add futex_priv_hash")
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250528085521.1938355-2-bigeasy@linutronix.de