checkpatch: warn on unnecessary line continuations

When the previous line is not a line continuation and the current line has
a line continuation but is not a #define, emit a warning.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>