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: force update backtraces for previously created FS

Update inode format version to 10, treat any previously created
inode as no backtrace. When inode with no backtrace is modified,
force update its backtrace.

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