Fix the method comment and return types of chunked_managed_vector's
reserve_partial() similar to chunked_vector's reserve_partial() as it
has the same issues mentioned in #19254. Also update the usage in the
chunked_managed_vector_test.
Signed-off-by: Lakshmi Narayanan Sreethar <lakshmi.sreethar@scylladb.com>
(cherry picked from commit d4f8b91bd6)