gve: unlink old napi only if page pool exists

gve: unlink old napi only if page pool exists

Commit de70981f295e ("gve: unlink old napi when stopping a queue using
queue API") unlinks the old napi when stopping a queue. But this breaks
QPL mode of the driver which does not use page pool. Fix this by checking
that there's a page pool associated with the ring.

Cc: stable@vger.kernel.org
Fixes: de70981f295e ("gve: unlink old napi when stopping a queue using queue API")
Reviewed-by: Joshua Washington <joshwash@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250317214141.286854-1-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>