Add opt.lg_extent_max_active_fit

When allocating from dirty extents (which we always prefer if available), large
active extents can get split even if the new allocation is much smaller, in
which case the introduced fragmentation causes high long term damage.  This new
option controls the threshold to reuse and split an existing active extent.  We
avoid using a large extent for much smaller sizes, in order to reduce
fragmentation.  In some workload, adding the threshold improves virtual memory
usage by >10x.

test/unit/mallctl.c

@@ -172,6 +172,7 @@ TEST_BEGIN(test_mallctl_opt) {
 	TEST_MALLCTL_OPT(bool, utrace, utrace);
 	TEST_MALLCTL_OPT(bool, xmalloc, xmalloc);
 	TEST_MALLCTL_OPT(bool, tcache, always);
+	TEST_MALLCTL_OPT(size_t, lg_extent_max_active_fit, always);
 	TEST_MALLCTL_OPT(size_t, lg_tcache_max, always);
 	TEST_MALLCTL_OPT(bool, prof, prof);
 	TEST_MALLCTL_OPT(const char *, prof_prefix, prof);