printk: Increase maximum CONFIG_LOG_BUF_SHIFT from 21 to 25

So I tried to some kernel debugging that produced a ton of kernel messages
on a big box, and wanted to save them all: but CONFIG_LOG_BUF_SHIFT maxes
out at 21 (2 MB).

Increase it to 25 (32 MB).

This does not affect any existing config or defaults.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>