file_data_sink_impl: Fix hang in close()
file_data_sink_impl: Fix hang in close()

file_data_sink_impl::wait(), called by both close() and flush(), takes
from _write_behind_sem but it will not signal it back if
_background_writes_done is failed, only when it is not failed. Because
of this, close() which follows flush() will block forever. Fix by
signalling in finally().

Fixes #133.

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