cleanup: Standardize the header guard define's name

At some point during early development, the <linux/cleanup.h> header
must have been named <linux/guard.h>, as evidenced by the header
guard name:

  #ifndef __LINUX_GUARDS_H
  #define __LINUX_GUARDS_H

It ended up being <linux/cleanup.h>, but the old guard name for
a file name that was never upstream never changed.

Do that now - and while at it, also use the canonical _LINUX prefix,
instead of the less common __LINUX prefix.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/171664113181.10875.8784434350512348496.tip-bot2@tip-bot2