linux-user: Convert signal number for FUTEX_FD

The val argument to FUTEX_FD is a signal number.  Convert to match
the host, as it will be converted back when the signal is delivered.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220829021006.67305-5-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>