ixgbe: fix potential RX buffer starvation for AF_XDP

When the RX rings are created they are also populated with buffers so
that packets can be received. Usually these are kernel buffers, but
for AF_XDP in zero-copy mode, these are user-space buffers and in this
case the application might not have sent down any buffers to the
driver at this point. And if no buffers are allocated at ring creation
time, no packets can be received and no interrupts will be generated so
the NAPI poll function that allocates buffers to the rings will never
get executed.

To rectify this, we kick the NAPI context of any queue with an
attached AF_XDP zero-copy socket in two places in the code. Once after
an XDP program has loaded and once after the umem is registered.  This
take care of both cases: XDP program gets loaded first then AF_XDP
socket is created, and the reverse, AF_XDP socket is created first,
then XDP program is loaded.

Fixes: d0bcacd0 ("ixgbe: add AF_XDP zero-copy Rx support")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>