btrfs-tree: clean up the fetch function's return set

btrfs-tree: clean up the fetch function's return set

Commit d32f31f411eeb1a8624b99dab223af69f0c8453e ("btrfs-tree: harden
`rlower_bound` against exceptional objects") passes the first btrfs item
in the result set that is above upper_bound up to `seek_backward`.
This is somewhat wasteful as `seek_backward` cannot use such a result.

Reverse that change in behavior, while keeping the rest of the other
commit.

This introduces a new case, where the search ioctl is producing items
that are above upper bound, but there are no items in the result set,
which continues looping until the end of the filesystem is reached.
Handle that by setting an explicit exit variable.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>