netmem: fix netmem comments

Trivial fix to a couple of outdated netmem comments. No code changes,
just more accurately describing current code.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250615203511.591438-1-almasrymina@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>