drm: Replace old pointer to new idr

In the Linux kernel, the following vulnerability has been resolved:

drm: Replace old pointer to new idr

Commit 5e28b7b94408 introduced a logical error by failing to replace the

newly generated IDR pointer to old id's pointer at the correct location

within the "change handle" logic; this resulted in the issue reported by

syzbot [1].

Specifically, the new IDR object pointer is intended to replace the original

id's pointer during the normal execution flow.

Additionally, an unnecessary conditional check for the ret exit path has

been removed.

[1]

!RB_EMPTY_ROOT(&prime_fpriv->dmabufs)

WARNING: drivers/gpu/drm/drm_prime.c:224 at drm_prime_destroy_file_private+0x48/0x60 drivers/gpu/drm/drm_prime.c:224, CPU#0: syz.0.17/5833

Call Trace:

drm_file_free.part.0+0x7e6/0xcc0 drivers/gpu/drm/drm_file.c:269

drm_file_free drivers/gpu/drm/drm_file.c:237 [inline]

drm_close_helper.isra.0+0x186/0x200 drivers/gpu/drm/drm_file.c:290

drm_release+0x1ab/0x360 drivers/gpu/drm/drm_file.c:438