netdevsim: Avoid allocation warnings triggered from user space

We need to suppress warnings from sily map sizes. Also switch
from GFP_USER to GFP_KERNEL_ACCOUNT, I'm pretty sure I misunderstood
the flags when writing this code.

Fixes: 395cacb5 ("netdevsim: bpf: support fake map offload")
Reported-by:  <syzbot+ad24705d3fd6463b18c6@syzkaller.appspotmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220726213605.154204-1-kuba@kernel.org