hyperv/hn: Reduce TCP segment aggregation limit for multiple RX rings

This mainly used to improve ACK timeliness when multiple RX rings
are enabled.

This value gives the best performance in both Azure and Hyper-V
environment, w/ both 10Ge and 40Ge using non-{INVARIANTS,WITNESS}
kernel.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5691

hyperv/hn: Factor out hn_set_lro_lenlim()

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5692

hyperv/hn: When short of mbufs on the RX path, don't spam the console.

Instead, increase the IQDROPS counter.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5693