xfstests: btrfs/011 improvement for compressed filesystems

xfstests: btrfs/011 improvement for compressed filesystems

Josef noticed that using /dev/zero to generate most of the test
data doesn't work if someone overrides the mount options to
enable compression. The test that performs a cancellation failed
because the replace operation was already finished when the
cancel request was executed.

Since /dev/urandom is too slow to generate multiple GB, the
way how the filesystem data is generated is completely changed
with this patch. Now /dev/urandom is used to generate one 1MB
file and this file is copied up to 2048 times. /dev/zero is no
longer used.

The runtime of the test is about the same as before. Compression
works now, online duplication will again cause issues, but
we don't have online duplication today.

Reported-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Reviewed-by: Jan Schmidt <list.xfs@jan-o-sch.net>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>