sglist_consume_uio truncates iov_len (size_t) to int - signedness confusion / data drop (dead code)

Summary

sglist_consume_uio (subr_sglist.c:389 int len; :404 len=iov->iov_len size_t->int truncation). iov_len multiple of 2^32 -> len=0 silently skipped (:405). iov_len>INT_MAX other -> len negative -> if(len>resid) signed/unsigned mix -> _sglist_append_buf huge length walk pages past iovec buffer. ZERO in-tree callers. Fix: size_t len.