nilfs2: convert __nilfs_msg to integrate the level and format

Reduce object size a bit by removing the KERN_<LEVEL> as a separate
argument and adding it to the format string.

Reduce overall object size by about ~.5% (x86-64 defconfig w/ nilfs2)

old:
$ size -t fs/nilfs2/built-in.a | tail -1
 191738	   8676	     44	 200458	  30f0a	(TOTALS)

new:
$ size -t fs/nilfs2/built-in.a | tail -1
 190971	   8676	     44	 199691	  30c0b	(TOTALS)

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1595860111-3920-3-git-send-email-konishi.ryusuke@gmail.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>