diag: Improve handling of IOCTLs

DIAG kernel driver interacts with user space processes using
IOCTLS. This change adds conditions to avoid potential integer
over/underflow, incorrect buffer copy.

CVE-2012-4220
CVE-2012-4221

Change-Id: Ic1e815051ae9544c911c9a5bd0c9218c1225f6d5
CRs-Fixed: 385352
CRs-Fixed: 385349
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>