Item values have been limited to a single value vector entry for a
while. They can't span 4K blocks in the segment format so they can't
cross kernel pages which are never smaller than 4K.
We don't need a helper to build a vector of their contents across
disjoint pages. This removes the last user of multi-element kvecs.
Signed-off-by: Zach Brown <zab@versity.com>