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: I63241c464dc6fcfac8e131d3d3b9f639a93de5bd
Signed-off-by: Fenil Panwala <quic_fpanwala@quicinc.com>
Signed-off-by: Madhab Sharma <quic_madhshar@quicinc.com>