coccinelle: api/kstrdup: fix coccinelle position

There is a typo in rule r2. Position p1 should be attached to kzalloc()
call.

Fixes: 29a36d4d ("scripts/coccinelle: improve the coverage of some semantic patches")
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>