ceph: handle cap import atomically

cap import messages are processed by both handle_cap_import() and
handle_cap_grant(). These two functions are not executed in the same
atomic context, so they can races with cap release.

The fix is make handle_cap_import() not release the i_ceph_lock when
it returns. Let handle_cap_grant() release the lock after it finishes
its job.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
ceph: introduce ceph_fill_fragtree()

Move the code that update the i_fragtree into a separate function.
Also add simple probabilistic test to decide whether the i_fragtree
should be updated

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
ceph: remember subtree root dirfrag's auth MDS

remember dirfrag's auth MDS when it's different from its parent inode's
auth MDS.

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