mds: reply dentry trace if LOOUPINO request has WANT_DENTRY flag

This 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
LOOKUPINO MDS request (with WANT_DENTRY flag set)

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