LU-14693 mdt: skip DLM when opening volatile files

LU-14693 mdt: skip DLM when opening volatile files

In mdt_reint_open(), when opening a volatile file skip taking a
MDS_INODELOCK_UPDATE lock on the parent directory.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I8ee89710f52e8097e1412897de91159702560e4a
Reviewed-on: https://review.whamcloud.com/43742
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>