LU-2875 mdt: use lu_name for rr_name and rr_tgt

LU-2875 mdt: use lu_name for rr_name and rr_tgt

In mdt_reint_record, use a struct lu_name for rr_name and
rr_tgt. Rename the rename target name rr_tgt to rr_tgt_name. Add a
function lu_name_is_valid() to check the validity of names. Remove
various assertions on the validity of names in MDT requests and
replace them with calls to mdt_reint_record() and returns of -EPROTO
on failure.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I6e0e207522a72eb8cec5d7918795c4aed5b9c4c2
Reviewed-on: http://review.whamcloud.com/7961
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>