e1000: Fix some issues in em_newitr()

• Load packet and byte counters exactly once, as they can be concurrently mutated.
• Rename bytes_packets to bytes_per_packet, which seems clearer.
• Use local variables that have the same types as the counter values, rather than truncating unsigned long to u32.

Reviewed by: kbowling
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D50416

(cherry picked from commit 731c145612dd6ffe457a562959a5c027acf13334)