Remove hpa_sec_batch_fill_extra and calculate nallocs automatically.

This change includes the following improvements:

- Remove the hpa_sec_batch_fill_extra parameter.
- Refactor the hpa_alloc() code and helper functions to be able to
  allocate more than one extent out of a single pageslab. This way
  we can amortize the per-pageslab costs (active bitmap iteration,
  pageslab metadata updates) across multiple extents.
- Decide on a min and max number of extents that will be allocated
  in hpa_alloc(). The code will try to allocate at least the min
  and allocate up to the max as long as we can allocate additional
  ones from the pageslab we already have, as additional allocations
  are relatively cheap.
- Add extent allocation distribution stats.
- Amend hpa_sec_integration.c unit test.

test/unit/mallctl.c

@@ -312,7 +312,6 @@ TEST_BEGIN(test_mallctl_opt) {
 	TEST_MALLCTL_OPT(size_t, hpa_sec_nshards, always);
 	TEST_MALLCTL_OPT(size_t, hpa_sec_max_alloc, always);
 	TEST_MALLCTL_OPT(size_t, hpa_sec_max_bytes, always);
-	TEST_MALLCTL_OPT(size_t, hpa_sec_batch_fill_extra, always);
 	TEST_MALLCTL_OPT(ssize_t, experimental_hpa_max_purge_nhp, always);
 	TEST_MALLCTL_OPT(size_t, hpa_purge_threshold, always);
 	TEST_MALLCTL_OPT(uint64_t, hpa_min_purge_delay_ms, always);