ext4: super.c: Update logging style using KERN_CONT

Recent commit require line continuing printks to use PR_CONT.

Update super.c to use KERN_CONT and use vsprintf extension %pV to
avoid a printk/vprintk/printk("\n") sequence as well.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>