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.  The previous reporting of the page size for frsize meant
that newer glibc and df would report a very small value for the fs size.

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

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Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>