netmap: Tell the compiler to avoid reloading ring indices

Per the removed comments these fields should be loaded only once, since
they can in principle be modified concurrently, though this would be a
violation of the userspace contract with netmap.

No functional change intended.

Reviewed by: vmaffione
MFC after: 1 week
Sponsored by: Zenarmor
Sponsored by: OPNsense
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D38061