vol: Remove O_EXCL|O_TRUNC combinations

A few places were specifying both O_EXCL and O_TRUNC to open().
O_TRUNC does not make any sense with O_EXCL, and doesn't do anything,
so remove O_TRUNC from these instances to make the code more clear.

Reviewed-on: http://gerrit.openafs.org/5832
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 4c679e380f697760de777d35fe057229bf2bf762)

Change-Id: Iae79f552d17d626bbf2a7c9428bc9c7bbf91fae9
Reviewed-on: http://gerrit.openafs.org/9433
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>