eth: bnxt: add support rx side device memory TCP

Currently, bnxt_en driver satisfies the requirements of the Device
memory TCP, which is HDS.
So, it implements rx-side Device memory TCP for bnxt_en driver.
It requires only converting the page API to netmem API.
`struct page` of agg rings are changed to `netmem_ref netmem` and
corresponding functions are changed to a variant of netmem API.

It also passes PP_FLAG_ALLOW_UNREADABLE_NETMEM flag to a parameter of
page_pool.
The netmem will be activated only when a user requests devmem TCP.

When netmem is activated, received data is unreadable and netmem is
disabled, received data is readable.
But drivers don't need to handle both cases because netmem core API will
handle it properly.
So, using proper netmem API is enough for drivers.

Device memory TCP can be tested with
tools/testing/selftests/drivers/net/hw/ncdevmem.
This is tested with BCM57504-N425G and firmware version 232.0.155.8/pkg
232.1.132.8.

Reviewed-by: Mina Almasry <[email protected]>
Tested-by: David Wei <[email protected]>
Signed-off-by: Taehee Yoo <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: TaeheeYoo