btr_exec_up(): Also ask given node.

btr_exec_up(): Also ask given node.

At the moment, the buffer tree walk starts at the parent of the given
node. Users of the btr API can not execute a command for a node unless
it has at least one child.

This unnecessary restriction is removed in this commit by letting
the tree walk start at the given node rather than its parent.
This actually simplifies the code a bit.