io_uring/net: use passed in 'len' in io_recv_buf_select()

io_uring/net: use passed in 'len' in io_recv_buf_select()

len is a pointer to the desired len, use that rather than grab it from
sr->len again. No functional changes as of this patch, but it does
prepare io_recv_buf_select() for getting passed in a value that differs
from sr->len.

Link: https://lore.kernel.org/io-uring/20250709203420.1321689-3-axboe@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>