Coccinelle: Add api/memdup_user

Use kmemdup_user rather than duplicating its implementation
This is a little bit restricted to reduce false positives

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