checkpatch: check for quoted strings broken across lines

checkpatch already makes an exception to the 80-column rule for quoted
strings, and Documentation/CodingStyle recommends not splitting quoted
strings across lines, because it breaks the ability to grep for the
string.  Rather than just permitting this, actively warn about quoted
strings split across lines.

Test case:

void context(void)
{
	struct { unsigned magic; const char *strdata; } foo[] = {
		{ 42, "these strings"
		      "do not produce warnings" },
		{ 256, "though perhaps"
		       "they should" },
	};
	pr_err("this string"
	       " should produce a warning\n");
	pr_err("this multi-line string\n"
	       "should not produce a warning\n");
	asm ("this asm\n\t"
	     "should not produce a warning");
}

Results of checkpatch on that test case:

WARNING: quoted string split across lines
+	       " should produce a warning\n");

total: 0 errors, 1 warnings, 15 lines checked

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Acked-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>