printk: ringbuffer: clear initial reserved fields

prb_reserve() will set some meta data values and leave others
uninitialized (or rather, containing the values of the previous
wrap). Simplify the API by always clearing out all the fields.
Only the sequence number is filled in. The caller is now
responsible for filling in the rest of the meta data fields.
In particular, for correctly filling in text and dict lengths.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200914123354.832-4-john.ogness@linutronix.de