ceph: add get_name() NFS export callback

Use the newly introduced LOOKUPNAME MDS request to connect child
inode to its parent directory.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>

diff --cc fs/ceph/export.c
Simple merge

diff --cc fs/ceph/inode.c
Simple merge

diff --cc fs/ceph/strings.c
Simple merge

diff --cc include/linux/ceph/ceph_fs.h
Simple merge