LU-6727 osd: auto locate BH_DXLock bit

LU-6727 osd: auto locate BH_DXLock bit

BH_DXLock use a fix code bit before. It may occupied by other one
silently. From lustre view, jdb2 & ext4 also use this defines, So
we use ext4's define to determine last value.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: I0fde8b461afec6acafe2708211761c3c60c529d7
Reviewed-on: http://review.whamcloud.com/15961
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>