igb: Expose RSS indirection table for ethtool

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>