eth: fbnic: Reuse RX mailbox pages

eth: fbnic: Reuse RX mailbox pages

Currently, the RX mailbox frees and reallocates a page for each received
message. Since FW Rx messages are processed synchronously, and nothing
hold these pages (unlike skbs which we hand over to the stack), reuse
the pages and put them back on the Rx ring. Now that we ensure the ring
is always fully populated we don't have to worry about filling it up
after partial population during init, either. Update
fbnic_mbx_process_rx_msgs() to recycle pages after message processing.

Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Link: https://patch.msgid.link/20260115003353.4150771-4-mohsin.bashr@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>