dout: use recursive mutex for dout

dout: use recursive mutex for dout

Using a recursive mutex for dout is desirable because it allows us to
survive situations like this:

> foo() { dout << "foo" << dendl; }
> dout(0) << foo() << dendl;

These cases should be fixed because they produce garbled output.
However, with this change, at least they will not cause deadlock.

Signed-off-by: Colin McCabe <colin.mccabe@dreamhost.com>