checkpatch: warn when using extern with function prototypes in .h files

Using the extern keyword on function prototypes is superfluous visual
noise so suggest removing it.

Using extern can cause unnecessary line wrapping at 80 columns and
unnecessarily long multi-line function prototypes.

Signed-off-by: Joe Perches <joe@perches.com>
Suggested-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>