LU-12570 mdt: request env for DT threads

LU-12570 mdt: request env for DT threads

as part of lock enqueue MDT thread can call ldlm_reclaim_full() to
cancel old unused LDLM locks and that scans all presented namespace
including OFD-originated (with extent locks). thus MDT ends with
calls into OFD code which needs own env marked with LCT_DT_THREAD.

Lustre-change: https://review.whamcloud.com/36179
Lustre-commit: 1f94d5eb2be4e921e909d8f18523dcab91bb6531

Test-Parameters: testlist=sanity,sanity,sanity,sanity envdefinitions=ONLY="134a",SHARED_KEY=true
Test-Parameters: testlist=sanity,sanity,sanity,sanity envdefinitions=ONLY="134a",SHARED_KEY=true
Test-Parameters: testlist=sanity,sanity,sanity,sanity envdefinitions=ONLY="134a",SHARED_KEY=true
Signed-off-by: Alexey Zhuravlev <bzzz@whamcloud.com>
Change-Id: I231b88159978bc3ce7a3fa0f27e57eb32137c343
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36312
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>