gve: Move the irq db indexes out of the ntfy block struct

Giving the device access to other kernel structs is not ideal.
Move the indexes into their own array and just keep pointers to
them in the ntfy block struct.

Signed-off-by: Catherine Sullivan <csully@google.com>
Signed-off-by: David Awogbemila <awogbemila@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>