block: relax check on sg gap
If the last bvec of the 1st bio and the 1st bvec of the next bio are physically contigious, and the latter can be merged to last segment of the 1st bio, we should think they don't violate sg gap(or virt boundary) limit. Both Vitaly and Dexuan reported lots of unmergeable small bios are observed when running mkfs on Hyper-V virtual storage, and performance becomes quite low. This patch fixes that performance issue. The same issue should exist on NVMe, since it sets virt boundary too. Reported-by: NVitaly Kuznetsov <vkuznets@redhat.com> Reported-by: NDexuan Cui <decui@microsoft.com> Tested-by: NDexuan Cui <decui@microsoft.com> Cc: Keith Busch <keith.busch@intel.com> Signed-off-by: NMing Lei <ming.lei@canonical.com> Signed-off-by: NJens Axboe <axboe@fb.com>
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