linux-user: Don't use CMSG_ALIGN(sizeof struct cmsghdr)

linux-user: Don't use CMSG_ALIGN(sizeof struct cmsghdr)

The Linux struct cmsghdr is already guaranteed to be sufficiently
aligned that CMSG_ALIGN(sizeof struct cmsghdr) is always equal
to sizeof struct cmsghdr. Stop doing the unnecessary alignment
arithmetic for host and target cmsghdr.

This follows kernel commit 1ff8cebf49ed9e9ca2 and brings our
TARGET_CMSG_* macros back into line with the kernel ones,
as well as making them easier to understand.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <1513345976-22958-3-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>