coccinelle: kfree_mismatch: drop vmalloc_exec

vmalloc_exec() was removed by commit 7a0e27b2a0ce ("mm: remove
vmalloc_exec").

So drop it from the rules.

No functional change.

Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>