fstream: fix reads when starting offset is nonzero
fstream: fix reads when starting offset is nonzero

If the starting offset is not 0 (modulo dma alignment), then the next read
will be short, and we didn't account for this.

Fix by aligning to buffer_size.  It's not clear whether we should align
to buffer_size or dma_alignment, but this is simpler.

koolhazz/seastar,hongliangzhao/seastar,syuu1228/seastar,joerg84/seastar,chunshengster/seastar,sjperkins/seastar,hongliangzhao/seastar,dreamsxin/seastar,leejir/seastar,xtao/seastar,shyamalschandra/seastar,dreamsxin/seastar,wildinto/seastar,avikivity/seastar,bowlofstew/seastar,leejir/seastar,flashbuckets/seastar,chunshengster/seastar,anzihenry/seastar,printedheart/seastar,bowlofstew/seastar,joerg84/seastar,cloudius-systems/seastar,flashbuckets/seastar,sjperkins/seastar,acbellini/seastar,tempbottle/seastar,stamhe/seastar,joerg84/seastar,dwdm/seastar,cloudius-systems/seastar,norcimo5/seastar,xtao/seastar,wildinto/seastar,tempbottle/seastar,jonathanleang/seastar,norcimo5/seastar,chunshengster/seastar,anzihenry/seastar,stamhe/seastar,scylladb/scylla-seastar,acbellini/seastar,anzihenry/seastar,jonathanleang/seastar,printedheart/seastar,norcimo5/seastar,dreamsxin/seastar,stamhe/seastar,leejir/seastar,linearregression/seastar,flashbuckets/seastar,cloudius-systems/seastar,kjniemi/seastar,tempbottle/seastar,linearregression/seastar,kjniemi/seastar,wildinto/seastar,avikivity/seastar,syuu1228/seastar,koolhazz/seastar,syuu1228/seastar,scylladb/scylla-seastar,scylladb/seastar,acbellini/seastar,sjperkins/seastar,scylladb/scylla-seastar,jonathanleang/seastar,shyamalschandra/seastar,koolhazz/seastar,scylladb/seastar,linearregression/seastar,dwdm/seastar,shyamalschandra/seastar,dwdm/seastar,kjniemi/seastar,printedheart/seastar,avikivity/seastar,hongliangzhao/seastar,scylladb/seastar,xtao/seastar,bowlofstew/seastar