bcache: Add __printf annotation to __bch_check_keys()

Make it possible for the compiler to verify the consistency of the
format string passed to __bch_check_keys() and the arguments that
should be formatted according to that format string.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>