OSD crashes with assert FAILED ceph_assert(mem_avail >= 0)

Issue

• Setting bluestore_cache_autotune=true while bluestore_cache_meta_ratio=0.8 and bluestore_cache_kv_ratio=0.2 are also set causes OSD to crash on startup.

• After bluestore_cache_autotune is enabled and the OSD is restarted the following line is seen in the backtrace. See diagnostic section for a complete backtrace example.

  /builddir/build/BUILD/ceph-16.2.10/src/common/PriorityCache.cc: 300: FAILED ceph_assert(mem_avail >= 0)