Build failure with Clang/LLVM 22 due to alloc-size diagnostic

Hi,

There is a new diagnostic, alloc-size, in clang of LLVM22 that warns if the
size given to a malloc is smaller than the size of the struct pointed to by
its destination - https://github.com/llvm/llvm-project/pull/150028
When we enable this option, in_mcast.c triggers this diagnostic, causing
the build to fail.

```
freebsd/sys/netinet/in_mcast.c:749:10: error: allocation of insufficient
size '40' for type 'struct ip_msource' with size '48' [-Werror,-Walloc-size]
  749 |                 nims = malloc(sizeof(struct in_msource),
M_INMFILTER,
      |                        ^
```

https://github.com/freebsd/freebsd-src/blob/stable/15/sys/netinet/in_mcast.c#L749
```
static int
imf_get_source(struct in_mfilter *imf, const struct sockaddr_in *psin,
    struct in_msource **plims)
{
          ...
struct ip_msource *ims, *nims;
  ...
nims = malloc(sizeof(struct in_msource), M_INMFILTER,
   M_NOWAIT | M_ZERO);
```

As the error message explained, the mismatch between struct ip_msource *
and malloc(sizeof(struct in_msource)) triggers the error.

However, when reading the source code carefully, it seems that *nims is
intentionally of type ip_msource instead of in_msource.
I would like to build with LLVM's alloc-size option enabled, but does
anyone have any good ideas on how to address this problem? Or would it be
better to report it as a false positive to LLVM? Though, I am aware that
there is a workaround to partially disable this option...