ext4: print ext4 mount option data_err=abort correctly

If data_err=abort option is specified for an ext3/ext4 mount,
/proc/mounts does show it as "(null)". This is caused by token2str()
returning NULL for Opt_data_err_abort (due to its pattern containing
'=').

Signed-off-by: Ales Novak <alnovak@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>