Linux: Fix a bug in CMSG_LEN()
Linux: Fix a bug in CMSG_LEN()

There was a bracing error in the transcription of the original C macro,
making it effectively equivalent to CMSG_SPACE().

(It would produce an even more weird result on systems where
CMSG_ALIGN(cmsghdr) is not a no-op -- but that seems a rather
hypothetical case, considering the definition of the structure, which
naturally aligns to sizeof size_t both on systems with sizeof size_t ==
sizeof int and with sizeof size_t == 2 * sizeof int...)

This bug resulted in undesired padding on systems where CMSG_ALIGN() is
not a total no-op. (I.e. x86_64, but not i386.)

servo/ipc-channel