xfstests: btrfs/013 regression test for prealloc with balance

We had a regression where we were not copying csums properly when balancing a
prealloc extent.  Unfortunately the way this showed up the most was with the
csum simply missing, which doesn't result in an error to userspace.  So I've
copied what generic/310 does and check dmesg for csum errors when the test
starts and then compare that count to the csum errors after the test finishes to
see if there was a problem.  This approach caught the error without my fix, and
then passed fine with my fix in place but with the previous errors still in
dmesg.  Thanks,

[rjohnston: changed test number to 13]

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Reviewed-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>

tests/btrfs/group

@@ -15,3 +15,4 @@
 010 auto quick
 011 auto
 012 auto
+013 auto quick