xfs_repair: don't use do_warn for normal log message

xfs_repair: don't use do_warn for normal log message

In some case, exit status of xfs_repair -n is different even for
the same file system when -v is specified or not. This patch fixes
this behavior.

If -v is specified, do_warn() is used in zero_log() for printing
a normal message. That makes the exit status to 1 though there
is no dirtiness in the file system.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
[sandeen: edit changelog for brevity]
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>