Revert "LU-4398 mdt: acquire an open lock for write or execute"

This is causing significant READ performance degradation in mdts: LU-5197

This reverts commit 708d85a652a77f85153790e6cca1b7a2b91947cf.

Change-Id: Ia591bed8a6f098ebad5bb8460c659516bcf24d97
Reviewed-on: http://review.whamcloud.com/10790
Tested-by: Jenkins
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Tested-by: Oleg Drokin <oleg.drokin@intel.com>