fstream: account for buffers dropped while closing stream
fstream: account for buffers dropped while closing stream

The original implementation increased the counter of needlessly read
bytes only when fstream::skip() was used. The same thing should be done
when the stream is closed and there are some unconsumed buffers.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

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