btrfs: better handle btrfs_printk() defaults

Commit 262c5e86 ("printk/btrfs: handle more message headers")
triggers:

    warning: `ratelimit' may be used uninitialized in this function

with gcc (4.1.2) and probably many other versions.  The code actually is
correct but a bit twisted.  Let's make it more straightforward and set
the default values at the beginning.

Link: http://lkml.kernel.org/r/20161213135246.GQ3506@pathway.suse.cz


Signed-off-by: Petr Mladek <pmladek@suse.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: David Sterba <dsterba@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>