When we're at our vnode limit, getnewvnode will call into the vnode LRU cache to free up vnodes. If the vnode we try to recycle is a ZFS vnode we end up, eventually, in zfs_rmnode. If the ZFS vnode we're recycling represents something with extended attributes, zfs_rmnode will call zfs_zget which will attempt to allocate another vnode. If the next vnode we try to recycle is also a ZFS vnode representing something with extended attributes we can recurse further. This ends up being unbounded and can end up overflowing the stack.
In order to avoid this, restructure zfs_rmnode to simply add the extended attribute directory's object ID to the unlinked set, thus not requiring the allocation of a vnode. We then schedule a task that calls zfs_unlinked_drain which will do the work of properly marking the vnodes for unlinking. zfs_unlinked_drain is also called on mount so these will be cleaned up there.