hyperv/hn: Improve sending performance

• Avoid main lock contention by trylock for if_start, if that fails, schedule TX taskqueue for if_start
• Don't do direct sending if the packet to be sent is large, e.g. TSO packet.

This change gives me stable 9.1Gbps TCP sending performance w/ TSO
over a 10Gbe directly connected network (the performance fluctuated
between 4Gbps and 9Gbps before this commit). It also improves non-
TSO TCP sending performance a lot.

Reviewed by: adrian, royger
Approved by: adrian (mentor)
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5074