kernel-doc: detect/prevent duplicate doc section names

I saw this problem recently.  With this kernel-doc:

 * Note: some important info
 *
 * Note: other important info

kernel-doc uses the "section name" (preceding the ':', like "Note") as a hash
key for storing the descriptive text ("blah important info").  It is (was)
possible to have duplicate (colliding) section names, without any kind of
warning or error.

kernel-doc happily used the latter descriptive text for all instances of
printing the <section-name> descriptive text and the former important info
was lost.

One way to "fix" this is to modify the kernel-doc comments, e.g.:

 * Note1: foo bar
 *
 * Note.2: blah zay

For now, kernel-doc will signal an error when it sees colliding section names
like this.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>