fix memory corruption in map_buffer TBUG-411 TQC-24

Change-Id: I509320ca4c31d6f5e3a30b3050a6055c83d3b7c7
Signed-off-by: Sylvain Trosset <trosset.sylvain@gmail.com>
Git-commit: 6814f81977d4f5b99ef8b55c8c1958af46f3ea01
Git-repo: https://code.google.com/p/tee-mobicore-driver.kernel/


Signed-off-by: Keith Fallows <keithf@codeaurora.org>
Acked-by: Tony Hamilton <tonyh@qti.qualcomm.com>