file_data_sink_impl: Make put() exception-safe
file_data_sink_impl: Make put() exception-safe

Allocation failure inside do_put() caused _write_behind_sem semaphore
to not get signalled, because the signalling was chained in the finally
block.

Fix by making do_put() noexcept and propagating failures via returned future.

Fixes #132.

syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar,kjniemi/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar,syuu1228/seastar,kjniemi/seastar,cloudius-systems/seastar,avikivity/seastar,cloudius-systems/seastar,kjniemi/seastar