igb: implement SIOCGHWTSTAMP ioctl

This patch adds support for the SIOCGHWTSTAMP ioctl which enables user
processes to read the current hwtstamp_config settings
non-destructively. Previously a process had to be privileged and could
only set values, it couldn't return what is currently set without
possibly overwriting the value.

This patch adds support for this new operation into igb by keeping a
shadow copy of the config in the adapter structure, which is returned
upon request.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>