io_uring/kbuf: fix signedness in this_len calculation

In the Linux kernel, the following vulnerability has been resolved:

io_uring/kbuf: fix signedness in this_len calculation

When importing and using buffers, buf->len is considered unsigned. However, buf->len is converted to signed int when committing. This can lead to unexpected behavior if the buffer is large enough to be interpreted as a negative value. Make min_t calculation unsigned.