eth: fbnic: Add basic Rx handling

Handle Rx packets with basic csum and Rx hash offloads.

NIC writes back to the completion ring a head buffer descriptor
(data buffer allocated from header pages), variable number of payload
descriptors (data buffers in payload pages), an optional metadata
descriptor (type 2) and finally the primary metadata descriptor
(type 3).

This format makes scatter support fairly easy - start gathering
the pages when we see head page, gather until we see the primary
metadata descriptor, do the processing. Use XDP infra to collect
the packet fragments as we traverse the descriptors. XDP itself
is not supported yet, but it will be soon.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://patch.msgid.link/172079942839.1778861.10509071985738726125.stgit@ahduyck-xeon-server.home.arpa


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