mds: fix MDCache::adjust_bounded_subtree_auth()

mds: fix MDCache::adjust_bounded_subtree_auth()

There are cases that need both create new bound and swallow intervening
subtree. For example: A MDS exports subtree A with bound B and imports
subtree B with bound C at the same time. The MDS crashes, exporting
subtree A fails, but importing subtree B succeed. During recovery, the
MDS may create new bound C and swallow subtree B.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Greg Farnum <greg@inktank.com>