ext4: don't show data=<mode> option if defaulted

Previously, mount -l would show data=<mode> even if the ext4 default
journaling mode was being used. Change this to be consistent with the
rest of the options.

Ext4 already did the right thing when the journaling mode being used
matched the one specified in the superblock's default mount options. The
reason it failed to do the right thing for the ext4 defaults is that,
when set, they were never included in sbi->s_def_mount_opt (unlike the
superblock's defaults, which were).

Signed-off-by: Tyson Nottingham <tgnottingham@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>