ceph: fix statvfs fr_size

Different versions of glibc are broken in different ways, but the short of
it is that for the time being, frsize should == bsize, and be used as the
multiple for the blocks, free, and available fields.  This mirrors what is
done for NFS.

Fixes http://tracker.ceph.com/issues/3793.

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