msm: adsprpc: Variable map may UAF due to race conditions

Variable map may pointing to the same buffer on race conditions
in functions fastrpc_internal_mmap, fastrpc_internal_munmap and,
fastrpc_internal_munmap_fd. Use mutex to avoid race conditions
on same buffer.

Change-Id: Ic51bce81a92b68516c9bdcf4f9955a06392d75e8
Acked-by: Krishnaiah Tadakamalla <ktadakam@qti.qualcomm.com>
Signed-off-by: Signed-off-by: Mohammed Nayeem Ur Rahman <mohara@codeaurora.org>