mds: evaluate stray when releasing inode/dentry's reference

Current method to purge stray inode is call MDCache::maybe_eval_stray()
after releasing a reference to the stray inode/dentry. It's difficult
to make this method work correct, because there are so many places that
can release reference.

This patch solves the issue by calling MDCache::maybe_eval_stray()
in MDSCacheObject::put(). This avoids adding code that calls
MDCache::maybe_eval_stray() to each place that releases reference.
To reduce virtual function call overhead, This patch introduces a new
state 'STATE_STRAY' for MDSCacheObject. The new state presents stray
inode/dentry. MDSCacheObject::put() calls MDCache::maybe_eval_stray()
only when releasing stray inode/dentry's reference.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: remove unnecessary MDCache::maybe_eval_stray() calls

Now we call MDCache::maybe_eval_stray() in MDSCacheObject::put().
So there is no need to call MDCache::maybe_eval_stray() after
releasing inode/dentry's refernece.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: don't trim stray inode from the cache.

don't trim stray inode from the cache, purge it instead. This ensures
the stary directories at minimum size.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: avoid fetching backtrace when purging stray

we save old data pools in both inode_backtrace_t::old_pools and
inode_t::old_pools. We have the inode in the cache when purging
stray, so no need to fetch backtrace to find old data pools.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: remove dirfrags when purging stray directory

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>