[fxfs] Change cmp_upper_bound to use (end,start) ordering.

Previously 0..100 and 50..100 would compare Equal under
cmp_upper_bound(). After this change, 0..100 < 50..100.

This change is in preparation for a subsequent change where we
will be reducing serialization overhead by allowing the use of
a serialized version of the key that maintains the same
lexicographical ordering as the deserialized key.

For this to work, we need cmp_upper_bound to share the same total
ordering as `memcmp` of the new serialized form of the keys would.

Change-Id: I1b88031fd46f582328c029a02329326f62969d27
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1604469
Commit-Queue: Aaron Drew <ripper@google.com>
Reviewed-by: Chris Suter <csuter@google.com>