mds: remove anchor table

use backtrace instead of anchors to find/open remote inodes

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

MDCache::discover_path() can replace MDCache::discover_ino() in
all cases, it also handles dirfrags better. This patch removes
discover ino related code.

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