xfstests: test ext4 statfs

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.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>