io_uring/rsrc: change ubuf->ubuf_end to length tracking

If we change it to tracking ubuf->start + ubuf->len, then we can reduce
the size of struct io_mapped_ubuf by another 4 bytes, effectively 8
bytes, as a hole is eliminated too.

This shrinks io_mapped_ubuf to 32 bytes.

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