nilfs2: remove whitespaces before quoted newlines

This kills the following checkpatch warnings:

 WARNING: unnecessary whitespace before a quoted newline
 #869: FILE: super.c:869:
 +     	           "remount to a different snapshot. \n",

 WARNING: unnecessary whitespace before a quoted newline
 #389: FILE: the_nilfs.c:389:
 +     	    printk(KERN_ERR "NILFS: too short segment. \n");

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>