Use the new vsprintf extension to avoid any possible
message interleaving.

Convert the #ifdef DEBUG block to a single pr_debug.
Signed-off-by: NJoe Perches <joe@perches.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>