osd: an interval can't go readwrite if its acting is empty

Let's not forget that min_size can be zero.

Fixes: #4159
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 4277265d99647c9fe950ba627e5d86234cfd70a9)
qa/run_xfstests.sh: use $TESTDIR instead of /tmp/cephtest

Signed-off-by: Sage Weil <sage@inktank.com>