btrfs-progs: mkfs: do not truncate the image when --rootdir is set

btrfs-progs: mkfs: do not truncate the image when --rootdir is set

With the rootdir option we try to guess the final size of the image and
fill it with zeros, preceded by truncation. After patch

"Btrfs-progs: Do not force mixed block group creation unless '-M' option
 is specified"

the misc test 002 will fail, because of the non-mixed mode. I think we
should not touch the image size (no change for block devices) and try to
fit into whatever is provided by user.

Signed-off-by: David Sterba <dsterba@suse.com>