binder: add barrier before enable preemption

"get_user" & "put_user" have a short piece of instructions, and without
the barrier compiler will optimize (i.e. delete) the "enable preemption".

Change-Id: Ice55a28ca0e38ea88127eb6dc35bf99a877bbbc7
Signed-off-by: Yi-wei Zhao <gbjc64@motorola.com>
Reviewed-on: http://gerrit.mot.com/857889


SME-Granted: SME Approvals Granted
SLTApproved: Slta Waiver <sltawvr@motorola.com>
Tested-by: Jira Key <jirakey@motorola.com>
Submit-Approved: Jira Key <jirakey@motorola.com>