iommu: Fix bug in __msm_map_iommu_common()

In error situations, iommu_meta remains NULL and msm_iommu_meta_put()
gets called with this NULL pointer. Fix this.

CRs-Fixed: 769233
Change-Id: I91c1e234c16d40af249a2d54faf6eae087281f5b
Signed-off-by: Neha Atri <natri@codeaurora.org>