ext2: avoid WARN() messages when failing to write to the superblock

This fixes a common warning reported by kerneloops.org

[Kernel summit hacking hour]
[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>