igc: reinit_locked() should be called with rtnl_lock

This commit applies to the igc_reset_task the same changes that
were applied to the igb driver in commit 024a8168 ("igb:
reinit_locked() should be called with rtnl_lock")
and fix possible race in reset subtask.

Fixes: 0507ef8a ("igc: Add transmit and receive fastpath and interrupt handlers")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>