This patch adds the ethtool callbacks necessary to change the RETA
indirection table from userspace.

In order to achieve this, we add the indirection table field (rss_indir_tbl)
in the board specific data structure (struct igb_adapter) to preserve the
values across hardware resets.

The indirection table must be initialized with default values in the
following cases:
	* at module init time
	* when the number of RX queues changes.
For this reason we add a new field (rss_indir_tbl_init) in igb_adapter
that keeps track of the number of RX queues. Whenever the number of RX
queues changes, the rss_indir_tbl is modified and initialized with default
values. The rss_indir_tbl_init is updated accordingly.

CC: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Laura Mihaela Vasilescu <laura.vasilescu@rosedu.org>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>