accel/rocket: fix UAF via dangling GEM handle in create_bo

Summary

In the Linux kernel, the following vulnerability has been resolved: accel/rocket: fix UAF via dangling GEM handle in create_bo rocket_ioctl_create_bo() inserts a GEM handle into the file's IDR via drm_gem_handle_create() early on, then performs several operations that can fail (sgt allocation, drm_mm insert, iommu_map). If any fail after the handle is live, the error path calls drm_gem_shmem_object_free() which kfree's the object without removing the handle from the IDR. This leaves a dangling handle pointing to freed slab memory. Any subsequent ioctl using that handle (PREP_BO, FINI_BO, SUBMIT) calls drm_gem_object_lookup() and dereferences freed memory (UAF). Fix by moving drm_gem_handle_create() to after all fallible operations succeed, matching the pattern used by panfrost, lima, and etnaviv. Also fix drm_mm_insert_node_generic() whose return value was silently overwritten by iommu_map_sgtable() on the next line. Add the missing error check. [tomeu: Move handle creation to the very end]