x86/mce: Rename mce_log to mce_log_buffer

It is confusing when staring at "struct mce_log mcelog" and then there's
also a function called mce_log(). So call the buffer what it is.

No functionality change.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/20170327093304.10683-4-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>