mds: update backtrace when old format inode is touched

We updated inode format version to 7 when introducing inode backtrace.
If we found a inode's format version < 7 when fetching the inode, set
the inode's backtrace_version to 0, to indicate the inode's backtrace
is out of data. When touching a inode with backtrace_version == 0, we
also update its backtrace.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #750 from ceph/wip-backtrace

Reviewed-by: Greg Farnum <greg@inktank.com>