unionfs: fix up VOP_UNLOCK use after flags stopped being supported

For the most part the code was passing the LK_RELEASE flag.
The 2 cases which did not use the VOP_UNLOCK_FLAGS macro.

This fixes a panic when stacking unionfs on top of e.g., tmpfs when
debug is enabled.

Note there are latent bugs which prevent unionfs from working with debug
regardless of this change.

PR: 243064
Reported by: Mason Loring Bliss