msm: adsprpc: use-after-free (UAF) in global maps

Currently, remote heap maps get added to the global list before the
fastrpc_internal_mmap function completes the mapping. Meanwhile, the
fastrpc_internal_munmap function accesses the map, starts unmapping, and
frees the map before the fastrpc_internal_mmap function completes,
resulting in a use-after-free (UAF) issue. Add the map to the list after
the fastrpc_internal_mmap function completes the mapping.

Change-Id: I72d24c7661244d1ff43454ce31dfc5e932090305
Acked-by: Abhishek Singh <abhishes@qti.qualcomm.com>
Signed-off-by: Santosh Sakore <quic_ssakore@quicinc.com>