checkpatch: returning errno typically should be negative

Add a (strict mode only) test to check for non-negative returns of what
appear to be errno values as the majority case these should indeed be
negative.

Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>