commitlog: fix size of a write used to zero a segment

Due to a bug the entire segment is written in one huge write of 32Mb.
The idea was to split it to writes of 128K, so fix it.

Fixes #5857

Message-Id: <20200220102939.30769-1-gleb@scylladb.com>
(cherry picked from commit df2f67626b)

db/commitlog/commitlog.cc

@@ -1323,7 +1323,7 @@ future<db::commitlog::segment_manager::sseg_ptr> db::commitlog::segment_manager:
                         std::vector<iovec> v;
                         v.reserve(n);
                         size_t m = 0;
-                        while (m < rem && n < max_write) {
+                        while (m < rem && n--) {
                             auto s = std::min(rem - m, buf_size);
                             v.emplace_back(iovec{ buf.get_write(), s});
                             m += s;