socket_msg_trunc: allocate a large enough buffer

[brooks]

Summary

Allocate 9000 bytes to match the largest requsted size. Add a check to
prevent the list of sizes and buffer size from getting out of sync
again.

Found with: CheriBSD

[markj]

It'd be a bit cleaner to allocate the buffer with malloc(), but this works.

tests/sys/kern/socket_msg_trunc.c

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