linux-user: Fix SNDCTL_DSP_MAP{IN, OUT}BUF ioctl definitions

Fix the SNDCTL_DSP_MAP{IN,OUT}BUF ioctl definitions so that they
refer to a suitably defined target struct layout rather than hardcoding
the ioctl number. This fixes complaints from the syscall_init()
consistency check when running an x86_64-to-x86_64 linux-user qemu.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
