sharded: map: copy mapper only once
sharded: map: copy mapper only once

It can be copied once using do_with
instead of being copied at each level's lambda
capture.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

syuu1228/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar