igb: check timestamp validity

Add a couple of checks to make sure timestamping is on and that the
timestamp value from DMA is valid. This avoids any functional issues
that could come from a misinterpreted time stamp.

One of the functions changed doesn't need a return value added because
there was no value in checking from the calling locations.

While here, fix a couple of reverse christmas tree issues next to
the code being changed.

Fixes: f56e7bba ("igb: Pull timestamp from fragment before adding it to skb")
Fixes: 9cbc948b

 ("igb: add XDP support")
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>