selftests: drv-net: add userns devmem RX test

selftests: drv-net: add userns devmem RX test

Add userns_devmem.py, which mirrors nk_devmem.py but places the netkit
guest in a netns whose owning user_ns is non-init. ncdevmem is ran there
via nsenter so the bind-rx call is issued with creds that hold
CAP_NET_ADMIN only in the child user_ns.

Without the preceding GENL_UNS_ADMIN_PERM patch the test fails at
bind-rx with EPERM, but with the patch the transfer completes and tests
pass.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260602-nl-prov-v2-2-ad721142c641@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>