checkpatch: add test for consecutive string fragments

Emit a warning when single line string coalescing occurs.

Code that uses compiler string concatenation on a single line like:

    printk("foo" "bar");

is generally better to read concatenated like:

    printk("foobar");

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>