btrfs-progs: convert: Fix a regression that ext2_save/image is not readonly

btrfs-progs: convert: Fix a regression that ext2_save/image is not readonly

The new convert treats the convert image as a normal file, without any
special flags and permissions.

This is different from original code:
1) Permission changed from 0400 to 0600
2) Inode lacks READONLY flag

This makes we can read-write mount the ext2 image and cause rollback
failure.

Follow old code behavior, use 0400 permission and add back READONLY
flag to fix it.

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