Coccinelle: limit memdup_user transformation to GFP_KERNEL case

Memdup_user encapsulates a memory allocation with the flag GFP_KERNEL, so
only allow this flag in the original code.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Michal Marek <mmarek@suse.com>