memshare: Prevent possible integer overflow

Prevent possible integer overflow by sanitizing the alloc request
size coming from the client against allottable amount of memory.

Change-Id: I74cb0f7b0808f20299586969fd5c810d44c3e576
Signed-off-by: Manoj Prabhu B <quic_bmanoj@quicinc.com>
Signed-off-by: Madhab Sharma <quic_madhshar@quicinc.com>