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 and fastrpc_internal_munmap,
use mutex to avoid race conditions on same buffer.

Change-Id: I96ed884c44a36f574677ba3ba189dfbf2ce3751d
Acked-by: Krishnaiah Tadakamalla <ktadakam@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>