22226: Update parent pointers when splicing a subtree.

22226: Update parent pointers when splicing a subtree.

When splicing a dirnode-rooted subtree S onto an existing dirnode D, D
itself remains in the filesystem tree, but D's children are replaced
with S's children.

Before this fix, S's immediate children were not getting their parent
pointers updated to point to D.  A subsequent Splice operation on D's
child would follow the child's parent pointer to S instead of D, try
to use S's FS(), which is nil, and panic.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>