os/bluestore: go beyond pinned onodes while trimming the cache.

os/bluestore: go beyond pinned onodes while trimming the cache.

One might face lack of cache trimming when there is a bunch of pinned entries on the top of Onode's cache LRU list. If these pinned entries stay in the state for a long time cache might start using too much memory causing OSD to go out of osd-memory-target limit. Pinned state tend to happen to osdmap onodes.
The proposed patch preserves last trim position in the LRU list (if it pointed to a pinned entry) and proceeds trimming from that position if it wasn't invalidated. LRU nature of the list enables to do that safely since no new entries appear above the previously present entry while it's not touched.

Fixes: https://tracker.ceph.com/issues/48729
Signed-off-by: Igor Fedotov <ifedotov@suse.com>