xfstests: test ext4 statfs

[Rich Johnston]

   xfstests: test ext4 statfs

     Calculating free blocks in ext[234] is surprisingly hard, since
     by default we report "bsd" style df which doesn't count filesystem
     "overhead" blocks as used.

     With a lot of code dedicated to sorting out what to report as
     free, things tend to go wrong surprisingly often.

     Here's a test to actually try to stop the next regression.  ;)

     NB: For bsddf, the kernel currently does not count journal blocks
     as overhead; it probably should.  But the test below looks to have
     the result within 1% of perfection, so it still passes even if
     the kernel doesn't count the journal against free blocks.

Eric,

This patch has been committed to git://oss.sgi.com/xfs/cmds/xfstests, 
master branch, commit 0b2ab695.

Thanks
--Rich