ceph: snapshot nfs re-export

To support snapshot nfs re-export, we need a way to lookup snapped
inode by file handle. For directory inode, snapped metadata are always
stored together with head inode. Client just need to pass vinodeno_t
to MDS. For non-directory inode, there can be multiple version of
snapped inodes and they can be stored in different dirfrags. Besides
vinodeno_t, client also need to tell mds from which dirfrag it got the
snapped inode.

Another problem of supporting snapshot nfs re-export is that there
can be multiple paths to access a snapped inode. For example:

  mkdir -p d1/d2/d3
  mkdir d1/.snap/s1

Paths 'd1/.snap/s1/d2/d3', 'd1/d2/.snap/_s1_<inode number of d1>/d3'
and 'd1/d2/d3/.snap/_s1_<inode number of d1>' are all reference to the
same snapped inode. For a given snapped inode, There is no convenient
way to get the first form and the second form paths. For simplicity,
ceph_get_parent() return snapdir for snapped directory inode.

Furthermore, client may access snapshot of deleted directory. For
example:

  mkdir -p d1/d2
  mkdir d1/.snap/s1
  open d1/.snap/s1/d2
  rm -rf d1/d2
  <nfs server restart>

The path constucted by ceph_get_parent() and ceph_get_name() is
'<inode of d2>/.snap/_s1_<inode number of d1>'. Futher lookup parent
of <inode of d2> will fail. To workaround this case, this patch uses
d_obtain_root() to get dentry for snapdir of deleted directory.
snapdir dentry has no DCACHE_DISCONNECTED flag set, reconnect_path()
stops when it reaches snapdir dentry.

Link: http://tracker.ceph.com/issues/22105
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>