Enhance bin run deallocation to avoid marking all pages as dirty, since the
dirty bits are already correct for all but the first page, due to the use of
arena_run_rc_{incr,decr}(). This tends to dramatically reduce the number of
pages that are marked dirty.
Modify arena_bin_run_size_calc() to assure that bin run headers never exceed
one page. In practice, this can't happen unless hard-coded constants (related
to RUN_MAX_OVRHD) are modified, but the dirty page tracking code assumes bin
run headers never extend past the first page, so it seems worth making this a
universally valid assumption.
952b7d192b