checkpatch: add check for line continuations in quoted strings

Add a warning for unterminated quoted strings with line continuations as
these frequently add unwanted whitespace.

Signed-off-by: Joe Perches <joe@perches.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>