mds: introduce LOOKUPNAME MDS request

The new MDS request is used for connecting a given inode to its
parent inode. It allows client to have efficient implementation of
get_rename() NFS export callback.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
client: fix Client::getcwd()

An recent commit made MDS not include dentry trace in LOOKUPPARENT
reply. It broke Client::getcwd. The fix is change getcwd() to use
LOOKUPNAME MDS request

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1385 from ceph/wip-nfs-export

mds: introduce LOOKUPNAME MDS request

Reviewed-by: Sage Weil <sage@inktank.com>