btrfs/012: adjust how we populate the fs to convert

btrfs/012: adjust how we populate the fs to convert

/lib/modules/$(uname -r)/ can have symlinks to the source tree where the
kernel was built from, which can have all sorts of stuff, which will
make the runtime for this test exceedingly long.  We're just trying to
copy some data into our tree to test with, we don't need the entire
devel tree of whatever we're doing.  Additionally VM's that aren't built
with modules will fail this test.

Update the test to use /etc, which will always exist.  Additionally use
timeout just in case there's large files or some other shenanigans so
the test doesn't run forever copying large amounts of files.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
[ fix use $TIMEOUT_PROG ]