checkpatch: look for c99 comments in ctx_locate_comment

Some checks look for comments around a specific function like
read_barrier_depends.

Extend the check to support both c89 and c90 comment styles.

	c89 /* comment */
or
	c99 // comment

For c99 comments, only look a 3 single lines, the line being scanned,
the line above and the line below the line being scanned rather than
the patch diff context.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Will Deacon <will@kernel.org>
Link: http://lkml.kernel.org/r/65cb075435d2f385a53c77571b491b2b09faaf8e.camel@perches.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>