sstables: don't repeat file_writer creation all the time
sstables: don't repeat file_writer creation all the time

When this code was originally written, we used to operate on a generic
output_stream. We created a file output stream, and then moved it into
the generic object.

Many patches and reworks later, we now have a file_writer object, but
that pattern was never reworked.

So in a couple of places we have something like this:

    f = file_object acquired by open_file_dma
    auto out = file_writer(std::move(f), 4096);
    auto w = make_shared<file_writer>(std::move(out));

The last statement is just totally redundant. make_shared can create
an object from its parameters without trouble, so we can just pass
the parameter list directly to it.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <c01801a1fdf37f8ea9a3e5c52cd424e35ba0a80d.1453219530.git.7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

duarten/scylla,scylladb/scylla,kjniemi/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,raphaelsc/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla