msm: adsprpc: Avoid race condition during map creation and free

Avoid race condition among concurrent fastrpc threads while
creating and removing memory mappings.

Change-Id: Ibf9ab0d4d19137804dbff4237a973895c410330c
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>