msm: adsprpc: Handle UAF in fastrpc internal munmap

Added reference count for contex map indicate memory under used
in remote call. And, this memory would not removed in internal
unmap to avoid UAF.

Change-Id: Ieb4ff6b298ff9c48953bc5b3539fdfe19a14b442
Acked-by: Santosh Sakore <ssakore@qti.qualcomm.com>
Signed-off-by: Santosh Sakore <quic_ssakore@quicinc.com>