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

Protect remote heap buffer list with spin lock while freeing
to avoid UAF in fastrpc_mmap_find() on a buffer that is freed
in fastrpc_mmap_free().

Change-Id: I524abf80087a5a53dfdf81c81ef34cd13f6a43cb
Acked-by: Maitreyi Gupta <maitreyi@qti.qualcomm.com>
Signed-off-by: jeyr <jeyr@codeaurora.org>