debugfs: fix set_inode_field so it can set the checksum field

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.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>