commitlog: Fix inner loop condition in allocation pre-fill

Fixes #8369

This was originally found (and fixed) by @gleb-cloudius, but the patch set with
the fix was reverted at some point, and the fix went away. Now the error remains
even in new, nice coroutine code.

We check the wrong var in the inner loop of the pre-fill path of
allocate_segment_ex, often causing us to generate giant writev:s of more or less
the whole file.  Not intended.

Closes #8370

db/commitlog/commitlog.cc

@@ -1440,7 +1440,7 @@ 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 && m < max_write) {
                         auto s = std::min(rem - m, buf_size);
                         v.emplace_back(iovec{ buf.get_write(), s});
                         m += s;