Fix mutator lock annotations for `nodes.cc`. am: c137762c96 am: 6cedd2078d am: a046d3731d

Original change: https://android-review.googlesource.com/c/platform/art/+/2427478

Change-Id: Ic20ef941bf983cf573b1e9399ca439934e9dd2a0
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>