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.

Fixes: ae98dbf4 ("io_uring/kbuf: add support for incremental buffer consumption")
Co-developed-by: Suoxing Zhang <aftern00n@qq.com>
Signed-off-by: Suoxing Zhang <aftern00n@qq.com>
Signed-off-by: Qingyue Zhang <chunzhennn@qq.com>
Link: https://lore.kernel.org/r/tencent_4DBB3674C0419BEC2C0C525949DA410CA307@qq.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>