LU-13439 lmv: qos stay on current MDT if less full

LU-13439 lmv: qos stay on current MDT if less full

Keep "space balanced" subdirectories on the parent MDT if it is less
full than average, since it doesn't make sense to select another MDT
which may occasionally be *more* full.  This also reduces random
"MDT jumping" and needless remote directories.

Reduce the QOS threshold for space balanced LMV layouts, so that the
MDTs don't become too imbalanced before trying to fix the problem.

Change the LUSTRE_OP_MKDIR opcode to be 1 instead of 0, so it can
be seen that a valid opcode has been stored into the structure.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iab34c7eade03d761aa16b08f409f7e5d69cd70bd