block: prevent an integer overflow in bvec_try_merge_hw_page

Reordered a check to avoid a possible overflow when adding len to bv_len.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20231204173419.782378-2-hch@lst.de


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