[Bug 285802] Fix MPASS(sa_len <= MLEN) compilation error with large MSIZE

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=285802

            Bug ID: 285802
           Summary: Fix MPASS(sa_len <= MLEN) compilation error with large
                    MSIZE
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: nakayamakenjiro@gmail.com

Created attachment 259208
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=259208&action=edit
A patch to fix MPASS(sa_len <= MLEN) compilation error with large MSIZE

In sys/kern/uipc_usrreq.c, there is an MPASS validation:

```
MPASS(from->sa_len <= MLEN);
```

However, if MSIZE is set to a larger value (e.g. 512), MLEN exceeds 255 — the
maximum value of an unsigned char — and the code fails to compile with strict
warnings enabled:

```
error: result of comparison of constant 480 with expression of type 'const
unsigned char' is always true
[-Werror,-Wtautological-constant-out-of-range-compare]
 1241 |         MPASS(from->sa_len <= MLEN);
```

In another part of the code, a similar check is guarded by a conditional, so
this validation should probably follow the same pattern.
In freebsd/sys/kern/uipc_sockbuf.c:

```
#if MSIZE <= 256
        if (asa->sa_len > MLEN)
                return (0);
#endif
```

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