xenbus: don't look up transaction IDs for ordinary writes

xenbus: don't look up transaction IDs for ordinary writes

This should really only be done for XS_TRANSACTION_END messages, or
else at least some of the xenstore-* tools don't work anymore.

Fixes: 0beef634b8 ("xenbus: don't BUG() on user mode induced condition")
Reported-by: Richard Schütz <rschuetz@uni-koblenz.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Richard Schütz <rschuetz@uni-koblenz.de>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
BugLink: http://bugs.launchpad.net/bugs/1611833
(cherry-picked from commit 9a035a40f7f3f6708b79224b86c5777a3334f7ea upstream)
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>