iommu: msm: Lock the meta data against map/unmap races

Reference counting is not enough to prevent races with map/unmap.
If a client tries to take an additional reference to an already
existing sg_table, it may be possible for an unmap to destroy the
reference while the map is still in progress. Prevent this race by
(properly) taking the meta lock around all lookup and reference calls.

Change-Id: Ic5de556db0d902e8b8f81b83b7897ad77940b269
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>