debugfs: fix set_inode_field so it can set the checksum field
Previously, setting the inode field was a no-op, since the library
function ext2fs_write_inode_full() would override whatever value was
set by debugfs. Use the new ext2fs_write_inode2() interface so we can
in fact set the checksum to a potentially wrong value. Also, ignore
the inode checksum failures if we are setting the checksum, and if the
checksum value is "calc", set the inode checksum to the correct value.