Remove duplicate dbufs accounting.

Since AVL already has embedded element counter, use dn_dbufs_count
only for dbufs not counted there (bonus buffers) and just add them.
This removes two atomics per dbuf life cycle.

According to profiler it reduces time spent by dbuf_destroy() inside
bottlenecked dbuf_evict_thread() from 13.36% to 9.20% of the core.

This counter is used only on illumos, so for FreeBSD it was just a
waste of time.

MFC after: 2 weeks