drm/shmem-helper: Add missing vunmap on error

The vmapping of dma-buf may succeed, but DRM SHMEM rejects the IOMEM
mapping, and thus, drm_gem_shmem_vmap_locked() should unvmap the IOMEM
before erroring out.

Cc: [e-mail]
Fixes: 49a3f51dfeee ("drm/gem: Use struct dma_buf_map in GEM vmap ops and convert GEM backends")
Signed-off-by: Dmitry Osipenko <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_gem_shmem_helper.c

···
 		ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
 		if (!ret) {
 			if (WARN_ON(map->is_iomem)) {
+				dma_buf_vunmap(obj->import_attach->dmabuf, map);
 				ret = -EIO;
 				goto err_put_pages;
 			}