test/send_recv: drop mutex on thread creation failure
test/send_recv: drop mutex on thread creation failure

Fixes: https://github.com/axboe/liburing/issues/379
Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing