LU-7057 utils: use stronger flags when opening volatile files

LU-7057 utils: use stronger flags when opening volatile files

Existing files with volatile-ish names may be accidentally opened by
llapi_create_volatile(). This is not the intent so disallow this
behavior by adding O_EXCL and O_NOFOLLOW to the flags to open().

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I4246ec7660b154ea2aeca6c28a4704ddd9d429f8
Reviewed-on: http://review.whamcloud.com/16126
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>