tree-wide: use CMSG_SPACE() (and not CMSG_LEN()) to allocate control buffers

tree-wide: use CMSG_SPACE() (and not CMSG_LEN()) to allocate control buffers

We need to use the CMSG_SPACE() macro to size the control buffers, not
CMSG_LEN(). The former is rounded up to next alignment boundary, the
latter is not. The former should be used for allocations, the latter for
encoding how much of it is actually initialized. See cmsg(3) man page
for details about this.

Given how confusing this is, I guess we don't have to be too ashamed
here, in most cases we actually did get this right.