netmem: add niov->type attribute to distinguish different net_iov types

netmem: add niov->type attribute to distinguish different net_iov types

Later patches in the series adds TX net_iovs where there is no pp
associated, so we can't rely on niov->pp->mp_ops to tell what is the
type of the net_iov.

Add a type enum to the net_iov which tells us the net_iov type.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20250508004830.4100853-2-almasrymina@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>