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: Ic3d5219898fcf23fb813eabe5fc2f187b397a472
Acked-by: Deepika Singh <dsi@qti.qualcomm.com>
Signed-off-by: Jeya R <jeyr@codeaurora.org>