Coccinelle: Use new comment format to explain kfree.cocci

Use new comment format to separate proposed commit message
and information about generated false positives

Signed-off-by: Nicolas Palix <npalix.work@gmail.com>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Michal Marek <mmarek@suse.cz>