btrfs-progs: convert-test: Check if the ext2_save/image is read only

btrfs-progs: convert-test: Check if the ext2_save/image is read only

Old convert codes uses both 0400 permission and INODE_READONLY flag to
make the converted ext2 image readonly.

While new convert treat the inode just as normal inode, with no special
inode flag and uses 0600 permission.
This makes user able to modify converted image unintentionally and make
rollback fails.

This test case will test the regression.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>