msm: adsprpc: Put upper limit on IOMMU mapping size

Place appropriate validation checks for IOMMU mapping size. Without
these checks, a mapping in an invalid kernel space could be created
and fail during access.

Change-Id: I0783834c09a5f7c37abfb29e2f098fbc86677ff2
Acked-by: Maitreyi Gupta <maitreyi@qti.qualcomm.com>
Signed-off-by: Mohammed Nayeem Ur Rahman <mohara@codeaurora.org>